LeetCode 032 Longest Valid Parentheses

题目描述：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.

代码如下：

class Solution {
public:
    int longestValidParentheses(string s) {

        int max_len = 0, last = -1; //last是上一次的')'的位置
        stack<int> lefts;       

        for(int i = 0; i < s.size(); i++){

            //s[i]为'('，则将i入栈
            if(s[i] == '(')
                lefts.push(i);

            //s[i]为')'
            else{
                if(lefts.empty())
                    last = i;
                else{
                    lefts.pop();

                    //若栈为空，则
                    if(lefts.empty())
                        max_len = max(max_len, i - last);
                    else
                        max_len = max(max_len, i - lefts.top());
                }
            }
        }

        return max_len;

    }
};