32. Longest Valid Parentheses

 1 class Solution {
 2     public int longestValidParentheses(String s) {
 3         if(s.length() == 0) return 0;
 4         char[] arr = s.toCharArray();
 5         Stack<Integer> stack = new Stack<>();
 6         int max = 0;
 7         for(int i = 0; i < s.length(); i++){
 8             char c = s.charAt(i);
 9             if(c == ‘(‘){
10                 stack.push(i);
11             }else if(c == ‘)‘ && !stack.isEmpty() && s.charAt(stack.peek()) == ‘(‘){
12                 stack.pop();
13             }else{
14                 stack.push(i);
15             }
16         }
17         if(stack.isEmpty()) return s.length();
18         int a = s.length();
19         while(!stack.isEmpty()){
20             int b = stack.pop();
21             max = Math.max(max, a-b-1);
22             a = b;
23         }
24         max = Math.max(a, max);
25         return max;
26         
27         
28     }
29 }