阿里面试程序上机题记录

• 1. 用两个栈(stacks)来实现一个队列(queue)，完成队列的push和pop操作。只能使用Stack的isEmpty()、push()与pop()三个操作。队列中的元素为int类型。

import java.util.Stack;

/**
 * <p>使用两个栈完成队列（先进先出）</p>
 */
public class Solution {

    private Stack<Integer> pushStack = new Stack<>();

    private Stack<Integer> popStack = new Stack<>();


    /**
     * 入栈
     */
    public void push(Integer value) {
        pushStack.push(value);
    }

    /**
     * 出栈
     */
    public Integer pop() {
        if (!popStack.isEmpty()) {
            return popStack.pop();
        }
        while (!pushStack.isEmpty()) {
            Integer temp = pushStack.pop();
            popStack.push(temp);
        }
        if (popStack.isEmpty()) {
            return null;
        }
        return popStack.pop();
    }

    /**
     * 判断栈是否为空
     */
    public boolean isEmpty() {
        return popStack.isEmpty() && pushStack.isEmpty();
    }

    /**
     * 测试入口
     */
    public static void main(String[] args) {
        Solution queue = new Solution();
        for (int i = 0; i < 100; i++) {
            queue.push(i);
        }
        System.out.println(queue.pop());
        System.out.println(queue.pop());
        while (!queue.isEmpty()) {
            System.out.println(queue.pop());
        }
    }
}

• 2. TinyURL是一个URL缩短服务，你可以输入一个URL像是https://leetcode.com/problems/design-tinyurl，然后这个服务会回传一个像是http://tinyurl.com/4e9iAk的短网址。请您为TinyURL设计其中的URL编码与解码函数。函数如何实现没有任何限制，但要确保一个URL要能编码为一个短网址，而此短网址要能解码成原本的网址。

package com.leno.jeep.simple.queue;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Random;

/**
 * <p>短url工具</p>
 *
 */
public class Codec {

    private HashMap<String, String> map = new HashMap<>();


    /**
     * 加密
     */
    public String encode(String longUrl) {
        for (HashMap.Entry<String, String> item : map.entrySet()) {
            if (item.getValue().equals(longUrl)) {
                return item.getKey();
            }
        }
        String shortUrl = "";
        while (map.get(shortUrl) != null) {
            shortUrl = getShortRandomUrl();
        }
        map.put(shortUrl, longUrl);
        return shortUrl;
    }

    /**
     * 解密
     */
    public String decode(String shortUrl) {
        return map.get(shortUrl);
    }

    /**
     * 随机url
     */
    public String getShortRandomUrl() {

        // 26个大写字母和10个数字
        String randomStr = "http://tinyurl.com/";
        Random random = new Random();
        for (int i = 0; i < 4; i++) {
            int tempI = random.nextInt(36);
            if (tempI < 10) {
                randomStr = randomStr + tempI;
            } else {
                char tempC = (char) ('A' + tempI - 10);
                randomStr = randomStr + tempC;
            }
        }
        return randomStr;
    }

    /**
     * 随机url
     */
    public String getLongRandomUrl() {

        // 26个大写字母和10个数字
        String randomStr = "https://leetcode.com/problems/design-tinyurl/";
        Random random = new Random();
        for (int i = 0; i < 20; i++) {
            int tempI = random.nextInt(36);
            if (tempI < 10) {
                randomStr = randomStr + tempI;
            } else {
                char tempC = (char) ('A' + tempI - 10);
                randomStr = randomStr + tempC;
            }
        }
        return randomStr;
    }

    public static void main(String[] args) {
        Codec demo = new Codec();
        List<String> urlList = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            urlList.add(demo.getLongRandomUrl());
        }

        for (String item : urlList) {
            System.out.println("原来的长链接：" + item);
            String shortUrl = demo.encode(item);
            System.out.println("短url：" + shortUrl);
            String longUrl = demo.decode(shortUrl);
            System.out.println("解码url：" + longUrl);
            System.out.println("=======================");
        }
    }

}