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.

解题思路：借助栈把所有能匹配的括号先弹出，栈内记录的是每个不能匹配的括号的下标。而每两个之间的都是能匹配的，所以求差值的最大值即为所求。

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