# 000032-Longest-Valid-Parentheses

### Problem

<https://leetcode.com/problems/longest-valid-parentheses/description/>

Given a string containing just the characters '(' and ')', return the length of the longest valid (well-formed) parentheses substring .

Example 1:

Input: s = "(()" Output: 2 Explanation: The longest valid parentheses substring is "()".

Example 2:

Input: s = ")()())" Output: 4 Explanation: The longest valid parentheses substring is "()()".

Example 3:

Input: s = "" Output: 0

Constraints:

0 <= s.length <= 3 \* 104 s\[i] is '(', or ')'.

### Solution

using stack to keep track index of every '('

```python
class Solution:
    def longestValidParentheses(self, s: str) -> int:
        max_len = 0
        # init with a start index 
        stack = [-1]
        for idx in range(len(s)):
            # when encounter of '(', put index into stack, keep track of index of '('
            if s[idx] == '(':
                stack.append(idx)
            else:
                # when encounter ')', pop out top index
                stack.pop()
                # if stack is empty (notice already init -1), indicates no valid cases at this point, continue 
                if not stack:
                    stack.append(idx)
                else: # if stack not empty, keep tracking max_length of the valid cases 
                    max_len = max(max_len, idx - stack[-1])

        return max_len
```


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