Merge pull request #11 from xchux/feature/155-min-stack

✨ (solution): Problem 155. Min Stack

Author: xchux

solutions/155. Min Stack/README.md

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+---
+comments: true
+difficulty: medium
+# Follow `Topics` tags
+tags:
+    - Stack
+    - Design
+---
+
+# [155. Min Stack](https://leetcode.com/problems/min-stack/description/)
+
+## Description
+
+Create a special type of stack called MinStack that, in addition to the usual stack operations, can also return the minimum element currently in the stack — all in constant time (O(1)).
+
+The MinStack class should support the following operations:
+
+1. `MinStack()`: Initializes a new instance of the stack.
+
+2. `void push(int val)`: Adds the given value `val` to the top of the stack.
+
+3. `void pop()`: Removes the top element from the stack.
+
+4. `int top()`: Returns the element currently at the top of the stack.
+
+5. `int getMin()`: Returns the smallest element in the stack at any given time.
+
+Make sure that all these methods run in constant time.
+
+
+**Example 1:**
+```
+Input:
+["MinStack","push","push","top","push","getMin","pop","top","getMin"]
+[[],[-1],[3],[],[-2],[],[],[],[]]
+
+Output: [null,null,3,null,-2,null,3,-1]
+
+Explanation:
+MinStack minStack = new MinStack();
+minStack.push(-1);
+minStack.push(3);
+minStack.top();    // return 3
+minStack.push(-2);
+minStack.getMin(); // return -2
+minStack.pop();
+minStack.top();    // return 3
+minStack.getMin(); // return -1
+```
+
+**Constraints:**
+
+* `-231 <= val <= 231 - 1`
+* Methods `pop`, `top` and `getMin` operations will always be called on non-empty stacks.
+* At most `3 * 10^4` calls will be made to `push`, `pop`, `top`, and `getMin`.
+
+## Solution
+
+Intuitively remove non-alphanumeric and change others to lowercase.
+
+```java
+```
+
+```python
+```
+
+## Complexity
+
+- Time complexity: $$O(1)$$
+<!-- Add time complexity here, e.g. $$O(n)$$ -->
+
+- Space complexity: $$O(n)$$
+<!-- Add space complexity here, e.g. $$O(n)$$ -->

solutions/155. Min Stack/Solution.java

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+import java.util.*;
+
+class Solution {
+    public boolean isValid(String s) {
+        Stack<Character> stack = new Stack<>();
+        for (char c : s.toCharArray()) {
+            if (c == '(' || c == '{' || c == '[') {
+                stack.push(c);
+            } else {
+                if (stack.isEmpty()) {
+                    return false;
+                }
+                char top = stack.pop();
+                if ((c == ')' && top != '(') || (c == '}' && top != '{') || (c == ']' && top != '[')) {
+                    return false;
+                }
+            }
+        }
+        return stack.isEmpty();
+    }
+}

solutions/155. Min Stack/Solution.py

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+class MinStack:
+
+    def __init__(self):
+        self.stack = []
+
+    def push(self, val: int) -> None:
+        top = self.stack[-1] if self.stack else -1
+        min_val = top[1] if isinstance(top, tuple) else float('inf')
+        if val < min_val:
+            min_val = val
+        self.stack.append((val, min_val))
+
+    def pop(self) -> None:
+        self.stack.pop()
+
+    def top(self) -> int:
+        return self.stack[-1][0] if self.stack else None
+
+    def getMin(self) -> int:
+        return self.stack[-1][1] if self.stack else None