Leetcode -- 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.

class Solution {
public:
    int longestValidParentheses(string s) {
        int n = s.size(),mval=0;
        vector<int> dp(n,0);
        for(int i=n-2;i>=0;--i)
        {
            if(s[i]=='(')
            {
                int j=i+1+dp[i+1];
                if(s[j]==')')
                {
                    dp[i] = dp[i+1]+2;
                    if(j+1<n)
                        dp[i]+=dp[j+1];
                }
                mval = max(mval,dp[i]);
            }
        }
        return mval;
    }
};