Leetcode 32: 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.

 

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public class Solution {
    public int LongestValidParentheses(string s) {
        int result = 0, cur = 0;
        var stack = new Stack<Tuple<char, int>>();
        bool lastPop = false;
        stack.Push(new Tuple<char, int>('*', -1));
        
        for (int j = 0; j < s.Length; j++)
        {
            if (s[j] == '(')
            {
                stack.Push(new Tuple<char, int>(s[j], j));
            }
            else
            {
                if (stack.Count > 0 && stack.Peek().Item1 == '(')
                {
                    stack.Pop();
                    result = Math.Max(result, j - stack.Peek().Item2);
                }
                else
                {
                    stack.Push(new Tuple<char, int>(s[j], j));
                }
            }
        }
        
        return result;
    }
}