32. Longest Valid Parentheses

 Longest Valid Parentheses
https://www.youtube.com/watch?v=M1Vw5Tk1rw4

 

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;
        
    }
}