Longest Valid Parentheses

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.

最长括号对，采用stack存储左括号的位置，遇到右括号时需要分情况处理，利用一个last变量来存储上一个完整括号对的左边括号的位置。

class Solution {
public:
    int longestValidParentheses(string s) {
        if(s.size()<2)
            return 0;
        int last =-1;
        stack<int> st;
        int  maxLen =0;
        for(int i =0; i<s.size();i++){
            if(s[i]=='(')
                st.push(i);
            else{
                if(st.size()==0)
                    last = i;
                else{
                    st.pop();
                    if(st.size()==0){
                        maxLen = max(maxLen, i-last);
                    }
                    else{
                        maxLen = max(maxLen,i-st.top());
                    }
                }
            }
        }
    return maxLen;
    }
};