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.

Solution: O(n).

class Solution {
public:
    int longestValidParentheses(string s) {
        stack<int> stk;
        for(int i = 0; i < s.size(); i++) {
            if(s[i] == '(') {
                stk.push(i);
            }
            else if(!stk.empty()){
                s[stk.top()] = '*';
                s[i] = '*';
                stk.pop();
            }
        }
        
        int part = 0, res = 0;
        for(int i = 0; i < s.size(); i++) {
            if(s[i] == '*') {
                part++;
            }
            else {
                res = max(res, part);
                part = 0;
            }
        }
        res = max(res, part);
        return res;
    }
};