[LeetCode] 32. Longest Valid Parentheses

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.

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