Longest Valid Parentheses - Leetcode

longestValidParentheses.java

/*
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) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(s.length() == 0) return 0;
        int curMax = 0;
        int absMax = 0;
        Stack<Character> stack = new Stack<Character>();
        
        for(int i = 0; i < s.length(); i++) {
            if(s.charAt(i) == '(') {
                stack.push('(');
            } else {
                if(stack.size() > 0) {
                    stack.pop();
                    curMax+=2;
                }
                if(curMax > absMax) {
                    absMax = curMax;
                    if(stack.size() == 0 && i < (s.length()-1) && s.charAt(i+1) == ')')
                        curMax = 0;
                }
            }
        }
        return absMax;
    }
}