leetcode 115: Longest Valid Parentheses

Longest Valid Parentheses Mar 1 '12

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
        //()(()
        int sz = s.length();
        Stack<Integer> stack = new Stack<Integer>();
        int max = 0;
        
        for(int i=0; i<sz; i++) {
            char c = s.charAt(i);
            if(c=='(') {
                stack.push(i);
            } else {
                if(!stack.isEmpty() && s.charAt(stack.peek()) == '(' ){
                    stack.pop();
                    
                    if(stack.isEmpty() ) {
                        max = Math.max(max, i+1);
                    } else {
                        int x = i-stack.peek();
                        max = Math.max(max, x);
                    }
                } else {
                    stack.push(i);
                }
            }
        }
        return max;
    }
}