Longest Valid Parentheses

Desicription

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

For "(()", the longest valid parentheses substring is "()", which has length = 2.

Another example is ")()())", where the longest valid parentheses substring is "()()", which has length = 4.

Solution

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class Solution {
public:
    int longestValidParentheses(string s) {
        stack<int>Q;
        for(int i = 0; s[i]; i++){
            if(Q.empty())
                Q.push(i);
            else{
                if(s[Q.top()] == '(' && s[i] == ')')
                    Q.pop();
                else
                    Q.push(i);
            }
        }
        if(Q.empty())
            return s.length();
        int res = 0;
        int right = s.length();
        int left = 0;
        while(!Q.empty()){
            left = Q.top();
            Q.pop();
            res = max(res, right - left - 1);
            right = left;
        }
        res = max(res, right);
        return res;
    }
};