32. Longest Valid Parentheses

Given a string containing just the characters '(' and ')', find the length of the longest valid (well-formed) parentheses substring.
Example 1:
Input: "(()"
Output: 2
Explanation: The longest valid parentheses substring is "()"
Example 2:
Input: ")()())"
Output: 4
Explanation: The longest valid parentheses substring is "()()"




https://www.youtube.com/watch?v=M1Vw5Tk1rw4&t=99s

http://bangbingsyb.blogspot.com/2014/11/leetcode-longest-valid-parentheses.html

// steps are according to youtube example 

class Solution {
    public int longestValidParentheses(String s) {
        if(s == null || s.length() < 2) return 0;
        int n = s.length();
        int max = 0;
        int leftMost = -1;
        
        Stack<Integer> stack = new Stack<>();
        for(int i = 0; i < n; i++){
            if(s.charAt(i) == '('){
                stack.push(i);
            }else{
                //  )
                if(stack.isEmpty()){
                    // util here, not a valid parenthese
                    leftMost = i;
                }else{
                    int j = stack.pop();
                    if(stack.isEmpty()){
                        max = Math.max(max, i - leftMost);
                    }else{
                        max = Math.max(max, i - stack.peek());
                    }
                }
            }
        }
        return max;
        
    }
}