[leetcode] Longest Valid Parentheses

问题描述

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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        stack<char> chs;
        stack<int>  top;
        char c;
        int length=s.size();
        int ret=0;
        chs.push('\0');
        top.push(-1);
        for(int i=0 ; i<length ; i++){
    		if(s[i]=='('){
				chs.push('(');
				top.push(i);
			}
			else{
				c=chs.top();
				if(c=='('){
					chs.pop();
					top.pop();
					int j=top.top();
					ret=max(ret,i-j);
				}else{
					chs.push(')');
					top.push(i);
				}
			}
		}
		return ret;
     }
};

这道题的主要思路在这一表达式ret=max(ret,i-j)；