LeetCode | Longest Valid Parentheses

题目

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.

分析

括号匹配自然就想到栈，为了能够计算出最长匹配长度，需要在栈中纪录'('的位置；

同时为了纪录“掐断”的位置，当')'无法匹配时，也放入栈中；

为了简化代码，栈初始化时放了个哨兵。

代码

import java.util.Stack;

public class LongestValidParentheses {
	public int longestValidParentheses(String s) {
		int max = 0;
		int N = s.length();
		Stack<Integer> stack = new Stack<Integer>();
		stack.push(-1);
		for (int i = 0; i < N; ++i) {
			if (s.charAt(i) == '(') {
				stack.push(i);
			} else {
				int index = stack.peek();
				if (index != -1 && s.charAt(index) == '(') {
					stack.pop();
					max = Math.max(max, i - stack.peek());
				} else {
					stack.push(i);
				}
			}
		}
		return max;
	}
}