Leetcode no. 32

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.

public class Solution {
    public int longestValidParentheses(String s) {
        Stack stack= new Stack();
        int max=0;
        int left=-1;
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i)=='(') stack.push(i);
            else {
                if (stack.isEmpty()) left=i;
                else {
                    stack.pop();
                    if (stack.isEmpty()) max=Math.max(max, i-left);
                    else max= Math.max(max, i-stack.peek());
                }
            }
        }
        return max;
    }
}