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：

使用动态规划。直接记录每一个位置的括号能否够跟前面的括号匹配，假设匹配，则记录为true

这里使用栈实现括号的匹配

int longestValidParentheses(string s) {
        int longestLen=0;
        if(s.size()<2)
            return 0;
        stack<int> st;
        vector<bool> dp;
        dp.resize(s.size(), false);
        
        for(int i=0; i<s.size(); i++){//实现动态规划
            if(s[i]=='('){
                st.push(i);
            }
            else if(st.size()){
                if(s[st.top()]=='('){
                    dp[st.top()]=true;
                    dp[i]=true;
                }
                st.pop();
            }
        }
        
        int localLong=0;
        int j;
        for(j=0; j<s.size(); j++){//计算最长匹配括号的长度
            if(dp[j]){
                localLong++;
                if(localLong>longestLen){
                    longestLen=localLong;
                }
            }
            else{
                localLong=0;
            }
        }
        
        if(longestLen%2==0)
            return longestLen;
        else
            return longestLen-1;
    }