[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.

Another string matching problem, this time I finish this problem in 30 minutes! and got ACCEPTED on my first submission. damn thats feels so good.
I discoverd an easy way to deal with it which I called it "mark&count" I'll explain it below, its easy and I think everyone could understand it.

init an array r with same size of string, containing int. 
iterate the string with index variable i
use stack for pair matching, that is, pushing the index in whenever encounter '(' and pop when ')' . 
when pop, mark all the position from stack.top() to i in r
after iterate finish, begin iterate r, count for longest marked position then thats the answer.

AC:

class Solution {
public:
    int longestValidParentheses(string s) {
            stack<int> t;
    int len = s.size();
    vector<int> r(len, 0);
    
    for (int i=0; i<len; i++){
        if (s[i] == '('){
            t.push(i);
        }
        else if (s[i] == ')'){
            if (!t.empty()){
                int top = t.top();
                t.pop();
                for (int j=top; j<i+1; j++){
                    r[j] = 1;
                }
            }
        }
    }
    int max=0,sum=0;
    for (int i=0; i<len; i++){
        if (r[i]){
            if (++sum > max){
                max = sum;
            }
        }else{
            sum = 0;
        }
    }
    return max;
    }
};