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.

Analyse: Consider scan the string from two sides. 

Runtime: 8ms

class Solution {
public:
    int longestValidParentheses(string s) {
        if(s.size() <= 1) return 0;
        
        int answer = 0, start = -1, depth = 0;
        
        for(int i = 0; i < s.size(); i++){
            if(s[i] == '(') depth++;
            else{
                depth--;
                if(depth < 0){
                    start = i;
                    depth = 0;
                }
                else if(depth == 0) answer = max(answer, i - start);
            }
        }
        
        depth = 0;
        start = s.size();
        for(int i = s.size() - 1; i >= 0; i--){
            if(s[i] == ')') depth++;
            else{
                depth--;
                if(depth < 0){
                    depth = 0;
                    start = i;
                }
                else if(depth == 0) answer = max(answer, start - i);
            }
        }
        return answer;
    }
};