Leetcode: Longest Valid Parentheses in C++

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.

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