#Leetcode# 32. Longest Valid Parentheses

https://leetcode.com/problems/longest-valid-parentheses/

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

Example 1:

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

Example 2:

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

代码：

class Solution {
public:
    int longestValidParentheses(string s) {
        int ans = 0, start = 0;
        stack<int> m;
        for (int i = 0; i < s.size(); ++i) {
            if (s[i] == '(') m.push(i);
            else if (s[i] == ')') {
                if (m.empty()) start = i + 1;
                else {
                    m.pop();
                    if(m.empty()) 
                        ans = max(ans, i - start + 1);
                    else ans = max(ans, i - m.top());
                }
            }
        }
        return ans;
    }
};

　　最长有效括号