✨ 128. Longest Consecutive Sequence

Author: xchux

solutions/128. Longest Consecutive Sequence/README.md

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+---
+comments: true
+difficulty: mediuum
+# Follow `Topics` tags
+tags:
+    - Array
+    - Hash Table
+    - Union Find
+---
+
+# [128. Longest Consecutive Sequence](https://leetcode.com/problems/longest-consecutive-sequence/description/)
+
+## Description
+Input is unsorted array integers, return the longest consecutive elements count.
+
+**Example 1:**
+```
+Input: nums = [9,2,41,3,5,4]
+Output: 4
+```
+The longest consecutive elements is `[2, 3, 4, 5]`. Therefore its count is 4.
+
+**Constraints:**
+
+`0 <= nums.length <= 10^5`
+
+`-10^9 <= nums[i] <= 10^9`
+
+
+## Solution
+For efficiency check num exist, conver nums to set.
+
+Then loop and check the consecutive count. 
+
+Avoid duplicate checking num which in the middle of consecutive sqeuence, skip the check when `num - 1` exist.
+
+```java
+class Solution {
+    public int longestConsecutive(int[] nums) {
+        Set<Integer> numSet = new HashSet<>();
+        for (int num : nums) {
+            numSet.add(num);
+        }
+        int longest = 0;
+        for (int num : nums) {
+            if (!numSet.contains(num - 1)) {
+                int length = 1;
+                while (numSet.contains(num + length)) {
+                    length++;
+                }
+                longest = Math.max(longest, length);
+            }
+        }
+        return longest;
+    }
+}
+```
+
+## Complexity
+
+- Time complexity: $O(n)$
+<!-- Add time complexity here, e.g. $$O(n)$$ -->
+
+- Space complexity: $O(n)$
+<!-- Add space complexity here, e.g. $$O(n)$$ -->
+

solutions/128. Longest Consecutive Sequence/Solution.java

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+class Solution {
+    public int longestConsecutive(int[] nums) {
+        Set<Integer> numSet = new HashSet<>();
+        for (int num : nums) {
+            numSet.add(num);
+        }
+        int longest = 0;
+        for (int num : nums) {
+            if (!numSet.contains(num - 1)) {
+                int length = 1;
+                while (numSet.contains(num + length)) {
+                    length++;
+                }
+                longest = Math.max(longest, length);
+            }
+        }
+        return longest;
+    }
+}