20-valid-parentheses

题目描述：

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. 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

代码解答：

package com.jack.algorithm;

import java.util.Stack;

/**
 * create by jack 2018/11/1
 *
 * @author jack
 * @date: 2018/11/1 23:01
 * @Description:
 * 括号匹配
 */
public class ValidParentheses {

    /**
     * 题目描述：
     * https://leetcode.com/problems/valid-parentheses/
     * @param s
     * @return
     */
    public static boolean isValid(String s) {
        if (s == null || s.isEmpty()) {
            return true;
        }
        int length = s.length();
        Stack<Character> stack = new Stack();
        for (int i=0;i<length;i++) {
            char c1 = s.charAt(i);
            if (c1 == '(' || c1 == '{' || c1 == '[') {
                stack.push(c1);
            } else {
                if (!stack.empty()) {
                    char c2 = stack.pop();
                    if (c1 ==')' && c2 == '(') {
                        continue;
                    }else if(c1 == '}' && c2 == '{'){
                        continue;
                    } else if(c1 == ']' && c2 =='['){
                        continue;
                    }else {
                        return false;
                    }
                } else {
                    return false;
                }
            }
        }
        if (stack.isEmpty()) {
            return true;
        } else {
            return false;
        }
    }

    public static void main(String[] args) {
        //String s = "()";
        //String s = "()[]{}";
        //String s = "(]";
        //String s = "([)]";
        String s = "{[]}";
        boolean flag = isValid(s);
        System.out.println("flag = "+flag);

    }
}

源码地址：

源码