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‎README.md

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‎problems/can-make-arithmetic-progression-from-sequence/README.md

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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--+----------------------------------------------------------------------+-->
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<!--|@author openset <[email protected]> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../countries-you-can-safely-invest-in "Countries You Can Safely Invest In")
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[Next >](../last-moment-before-all-ants-fall-out-of-a-plank "Last Moment Before All Ants Fall Out of a Plank")
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## [1502. Can Make Arithmetic Progression From Sequence (Easy)](https://leetcode.com/problems/can-make-arithmetic-progression-from-sequence "判断能否形成等差数列")
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<p>Given an array of numbers <code>arr</code>.&nbsp;A sequence of numbers is called an arithmetic progression&nbsp;if the difference between any two consecutive elements is the same.</p>
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<p>Return <code>true</code>&nbsp;if the array can be rearranged to form an arithmetic progression, otherwise, return <code>false</code>.</p>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> arr = [3,5,1]
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<strong>Output:</strong> true
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<strong>Explanation: </strong>We can reorder the elements as [1,3,5] or [5,3,1] with differences 2 and -2 respectively, between each consecutive elements.
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</pre>
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<p><strong>Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> arr = [1,2,4]
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<strong>Output:</strong> false
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<strong>Explanation: </strong>There is no way to reorder the elements to obtain an arithmetic progression.
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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>2 &lt;= arr.length &lt;= 1000</code></li>
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<li><code>-10^6 &lt;= arr[i] &lt;= 10^6</code></li>
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</ul>
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### Related Topics
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[[Sort](../../tag/sort/README.md)]
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[[Array](../../tag/array/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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Consider that any valid arithmetic progression will be in sorted order.
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</details>
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<details>
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<summary>Hint 2</summary>
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Sort the array, then check if the differences of all consecutive elements are equal.
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</details>

‎problems/count-numbers-with-unique-digits/README.md

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&nbsp; excluding <code>11,22,33,44,55,66,77,88,99</code>
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</pre>
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</div>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>0 &lt;= n &lt;= 8</code></li>
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</ul>
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### Related Topics
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[[Math](../../tag/math/README.md)]

‎problems/count-submatrices-with-all-ones/README.md

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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--+----------------------------------------------------------------------+-->
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<!--|@author openset <[email protected]> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../last-moment-before-all-ants-fall-out-of-a-plank "Last Moment Before All Ants Fall Out of a Plank")
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[Next >](../minimum-possible-integer-after-at-most-k-adjacent-swaps-on-digits "Minimum Possible Integer After at Most K Adjacent Swaps On Digits")
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## [1504. Count Submatrices With All Ones (Medium)](https://leetcode.com/problems/count-submatrices-with-all-ones "统计全 1 子矩形")
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<p>Given a&nbsp;<code>rows * columns</code>&nbsp;matrix <code>mat</code> of ones and zeros, return how many&nbsp;<strong>submatrices</strong> have all ones.</p>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> mat = [[1,0,1],
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&nbsp; [1,1,0],
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&nbsp; [1,1,0]]
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<strong>Output:</strong> 13
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<strong>Explanation:
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</strong>There are <b>6</b> rectangles of side 1x1.
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There are <b>2</b> rectangles of side 1x2.
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There are <b>3</b> rectangles of side 2x1.
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There is <b>1</b> rectangle of side 2x2.
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There is <b>1</b> rectangle of side 3x1.
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Total number of rectangles = 6 + 2 + 3 + 1 + 1 = <strong>13.</strong>
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</pre>
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<p><strong>Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> mat = [[0,1,1,0],
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&nbsp; [0,1,1,1],
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&nbsp; [1,1,1,0]]
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<strong>Output:</strong> 24
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<strong>Explanation:</strong>
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There are <b>8</b> rectangles of side 1x1.
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There are <b>5</b> rectangles of side 1x2.
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There are <b>2</b> rectangles of side 1x3.
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There are <b>4</b> rectangles of side 2x1.
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There are <b>2</b> rectangles of side 2x2.
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There are <b>2</b> rectangles of side 3x1.
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There is <b>1</b> rectangle of side 3x2.
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Total number of rectangles = 8 + 5 + 2 + 4 + 2 + 2 + 1 = 24<strong>.</strong>
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</pre>
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<p><strong>Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> mat = [[1,1,1,1,1,1]]
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<strong>Output:</strong> 21
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</pre>
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<p><strong>Example 4:</strong></p>
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<pre>
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<strong>Input:</strong> mat = [[1,0,1],[0,1,0],[1,0,1]]
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<strong>Output:</strong> 5
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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>1 &lt;= rows&nbsp;&lt;= 150</code></li>
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<li><code>1 &lt;= columns&nbsp;&lt;= 150</code></li>
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<li><code>0 &lt;= mat[i][j] &lt;= 1</code></li>
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</ul>
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### Related Topics
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[[Dynamic Programming](../../tag/dynamic-programming/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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For each row i, create an array nums where: if mat[i][j] == 0 then nums[j] = 0 else nums[j] = nums[j-1] +1.
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</details>
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<details>
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<summary>Hint 2</summary>
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In the row i, number of rectangles between column j and k(inclusive) and ends in row i, is equal to SUM(min(nums[j, .. idx])) where idx go from j to k. Expected solution is O(n^3).
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</details>

‎problems/countries-you-can-safely-invest-in/README.md

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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--+----------------------------------------------------------------------+-->
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<!--|@author openset <[email protected]> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../design-a-file-sharing-system "Design a File Sharing System")
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[Next >](../can-make-arithmetic-progression-from-sequence "Can Make Arithmetic Progression From Sequence")
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## [1501. Countries You Can Safely Invest In (Medium)](https://leetcode.com/problems/countries-you-can-safely-invest-in "")
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‎problems/countries-you-can-safely-invest-in/mysql_schemas.sql

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Create table If Not Exists Person (id int, name varchar(15), phone_number varchar(11));
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Create table If Not Exists Country (name varchar(15), country_code varchar(3));
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Create table If Not Exists Calls (caller_id int, callee_id int, duration int);
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Truncate table Person;
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insert into Person (id, name, phone_number) values ('3', 'Jonathan', '051-1234567');
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insert into Person (id, name, phone_number) values ('12', 'Elvis', '051-7654321');
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insert into Person (id, name, phone_number) values ('1', 'Moncef', '212-1234567');
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insert into Person (id, name, phone_number) values ('2', 'Maroua', '212-6523651');
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insert into Person (id, name, phone_number) values ('7', 'Meir', '972-1234567');
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insert into Person (id, name, phone_number) values ('9', 'Rachel', '972-0011100');
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Truncate table Country;
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insert into Country (name, country_code) values ('Peru', '051');
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insert into Country (name, country_code) values ('Israel', '972');
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insert into Country (name, country_code) values ('Morocco', '212');
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insert into Country (name, country_code) values ('Germany', '049');
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insert into Country (name, country_code) values ('Ethiopia', '251');
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Truncate table Calls;
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insert into Calls (caller_id, callee_id, duration) values ('1', '9', '33');
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insert into Calls (caller_id, callee_id, duration) values ('2', '9', '4');
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insert into Calls (caller_id, callee_id, duration) values ('1', '2', '59');
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insert into Calls (caller_id, callee_id, duration) values ('3', '12', '102');
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insert into Calls (caller_id, callee_id, duration) values ('3', '12', '330');
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insert into Calls (caller_id, callee_id, duration) values ('12', '3', '5');
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insert into Calls (caller_id, callee_id, duration) values ('7', '9', '13');
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insert into Calls (caller_id, callee_id, duration) values ('7', '1', '3');
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insert into Calls (caller_id, callee_id, duration) values ('9', '7', '1');
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insert into Calls (caller_id, callee_id, duration) values ('1', '7', '7');

‎problems/design-a-file-sharing-system/README.md

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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--+----------------------------------------------------------------------+-->
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<!--|@author openset <[email protected]> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../max-value-of-equation "Max Value of Equation")
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[Next >](../countries-you-can-safely-invest-in "Countries You Can Safely Invest In")
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## [1500. Design a File Sharing System (Medium)](https://leetcode.com/problems/design-a-file-sharing-system "")
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### Related Topics
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[[Array](../../tag/array/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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Try to solve it by keeping for each file chunk, the users who have this chunk.
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</details>
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<details>
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<summary>Hint 2</summary>
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Try to solve it by keeping all the users in the system with their owned chunks, and when you request a chunk, check all users for it.
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</details>

‎problems/employee-importance/README.md

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## [690. Employee Importance (Easy)](https://leetcode.com/problems/employee-importance "员工的重要性")
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<p>You are given a data structure of employee information, which includes the employee&#39;s <b>unique id</b>, his <b>importance value</b> and his <b>direct</b> subordinates&#39; id.</p>
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<p>You are given a data structure of employee information, which includes the employee&#39;s <b>unique id</b>, their&nbsp;<b>importance value</b> and their&nbsp;<b>direct</b> subordinates&#39; id.</p>
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<p>For example, employee 1 is the leader of employee 2, and employee 2 is the leader of employee 3. They have importance value 15, 10 and 5, respectively. Then employee 1 has a data structure like [1, 15, [2]], and employee 2 has [2, 10, [3]], and employee 3 has [3, 5, []]. Note that although employee 3 is also a subordinate of employee 1, the relationship is <b>not direct</b>.</p>
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<p>Now given the employee information of a company, and an employee id, you need to return the total importance value of this employee and all his subordinates.</p>
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<p>Now given the employee information of a company, and an employee id, you need to return the total importance value of this employee and all their&nbsp;subordinates.</p>
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<p><b>Example 1:</b></p>
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<li>The maximum number of employees won&#39;t exceed 2000.</li>
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</ol>
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<p>&nbsp;</p>
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### Related Topics
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[[Depth-first Search](../../tag/depth-first-search/README.md)]
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[[Breadth-first Search](../../tag/breadth-first-search/README.md)]

‎problems/last-moment-before-all-ants-fall-out-of-a-plank/README.md

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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--+----------------------------------------------------------------------+-->
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<!--|@author openset <[email protected]> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../can-make-arithmetic-progression-from-sequence "Can Make Arithmetic Progression From Sequence")
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[Next >](../count-submatrices-with-all-ones "Count Submatrices With All Ones")
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## [1503. Last Moment Before All Ants Fall Out of a Plank (Medium)](https://leetcode.com/problems/last-moment-before-all-ants-fall-out-of-a-plank "所有蚂蚁掉下来前的最后一刻")
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<p>We have a wooden&nbsp;plank of the length <code>n</code> <strong>units</strong>. Some ants are walking on the&nbsp;plank, each ant moves with speed <strong>1 unit per second</strong>. Some of the ants move to the <strong>left</strong>, the other move to the <strong>right</strong>.</p>
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<p>When two ants moving in two <strong>different</strong> directions meet at some point, they change their directions and continue moving again. Assume changing directions doesn&#39;t take any additional time.</p>
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<p>When an ant reaches <strong>one end</strong> of the plank at a time <code>t</code>, it falls out of the plank imediately.</p>
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<p>Given an integer <code>n</code> and two integer arrays <code>left</code> and <code>right</code>, the positions of the ants moving to the left and the right.&nbsp;Return <em>the&nbsp;moment</em> when the last ant(s) fall out of the plank.</p>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<img alt="" src="https://assets.leetcode.com/uploads/2020/06/17/ants.jpg" style="width: 450px; height: 610px;" />
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<pre>
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<strong>Input:</strong> n = 4, left = [4,3], right = [0,1]
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<strong>Output:</strong> 4
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<strong>Explanation:</strong> In the image above:
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-The ant at index 0 is named A and going to the right.
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-The ant at index 1 is named B and going to the right.
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-The ant at index 3 is named C and going to the left.
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-The ant at index 4 is named D and going to the left.
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Note that the last moment when an ant was on the plank is t = 4 second, after that it falls imediately out of the plank. (i.e. We can say that at t = 4.0000000001, there is no ants on the plank).
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</pre>
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<p><strong>Example 2:</strong></p>
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<img alt="" src="https://assets.leetcode.com/uploads/2020/06/17/ants2.jpg" style="width: 639px; height: 101px;" />
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<pre>
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<strong>Input:</strong> n = 7, left = [], right = [0,1,2,3,4,5,6,7]
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<strong>Output:</strong> 7
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<strong>Explanation:</strong> All ants are going to the right, the ant at index 0 needs 7 seconds to fall.
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</pre>
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<p><strong>Example 3:</strong></p>
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<img alt="" src="https://assets.leetcode.com/uploads/2020/06/17/ants3.jpg" style="width: 639px; height: 100px;" />
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<pre>
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<strong>Input:</strong> n = 7, left = [0,1,2,3,4,5,6,7], right = []
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<strong>Output:</strong> 7
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<strong>Explanation:</strong> All ants are going to the left, the ant at index 7 needs 7 seconds to fall.
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</pre>
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<p><strong>Example 4:</strong></p>
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<pre>
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<strong>Input:</strong> n = 9, left = [5], right = [4]
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<strong>Output:</strong> 5
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<strong>Explanation:</strong> At t = 1 second, both ants will be at the same intial position but with different direction.
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</pre>
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<p><strong>Example 5:</strong></p>
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<pre>
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<strong>Input:</strong> n = 6, left = [6], right = [0]
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<strong>Output:</strong> 6
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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>1 &lt;= n &lt;= 10^4</code></li>
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<li><code>0 &lt;= left.length &lt;= n + 1</code></li>
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<li><code>0 &lt;= left[i] &lt;= n</code></li>
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<li><code>0 &lt;= right.length &lt;= n + 1</code></li>
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<li><code>0 &lt;= right[i] &lt;= n</code></li>
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<li><code>1 &lt;= left.length + right.length &lt;= n + 1</code></li>
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<li>All values of <code>left</code> and <code>right</code> are unique, and each value can appear <strong>only in one</strong> of the two arrays.</li>
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</ul>
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### Related Topics
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[[Brainteaser](../../tag/brainteaser/README.md)]
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[[Array](../../tag/array/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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The ants change their way when they meet is equivalent to continue moving without changing their direction.
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</details>
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<details>
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<summary>Hint 2</summary>
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Answer is the max distance for one ant to reach the end of the plank in the facing direction.
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</details>

‎problems/lowest-common-ancestor-of-a-binary-search-tree/README.md

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</pre>
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<p>&nbsp;</p>
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<p><strong>Note:</strong></p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li>All of the nodes&#39; values will be unique.</li>

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