LeetCode刷题：20. Valid Parentheses
原题链接：https://leetcode.com/problems/valid-parentheses/
Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
An input string is valid if:
Open brackets must be closed by the same type of brackets. Open brackets must be closed in the correct order. Note that an empty string is also considered valid.
Example 1:
Input: "()" Output: true Example 2:
Input: "()[]{}" Output: true Example 3:
Input: "(]" Output: false Example 4:
Input: "([)]" Output: false Example 5:
Input: "{[]}" Output: true
算法设计
package com.bean.algorithm.basic; import java.util.Stack; public class ValidParentheses { public boolean isValid(String s) { if (s == "" || s.isEmpty()) return true; Stack stack = new Stack(); for (int index = 0; index < s.length(); index++) { char ch = s.charAt(index); if (ch == '(' || ch == '[' || ch == '{') { stack.push(ch); } else { char expected; if (ch == ')') expected = '('; else if (ch == ']') expected = '['; else expected = '{'; if (stack.isEmpty()) { return false; } char actual = (char) stack.pop(); if (actual != expected) { return false; } } } return stack.isEmpty(); } public static void main(String[] args) { ValidParentheses vp=new ValidParentheses(); //String input="()[]{}"; String input="([)]"; boolean flag=vp.isValid(input); System.out.println("flag is: "+flag); } }程序运行结果：
当 input = “()[]{}”时，返回 flag = true；
当 input = “([)]”时，返回 flag = false
