P59、面试题7：用两个栈实现队列

题目：用两个栈实现一个队列。队列声明如下，请实现它的两个函数appendTail和deleteHead，分别完成在队列为插入结点和在队列头部删除结点的功能。

stack1专门用于插入数据，stack2专门用于删除数据，如果stack2没有数据了，则将stack1的数据转移到stack2中，直到两个栈都没有数据。 

测试用例：

1）往空的队列里添加、删除元素

2）往非空的队列里添加、删除元素

3）连续删除元素直到队列为空

 

Queue类：

package com.yyq;

import java.util.Stack;

/**
 * Created by Administrator on 2015/9/8.
 */
public class Queue<T> {
    private T value;
    public static final Stack stack1 = new Stack();
    public static final Stack stack2 = new Stack();
    public T getValue() {
        return value;
    }

    public void setValue(T value) {
        this.value = value;
    }

    public Queue(){
    }

    public void appendTail(T value){
        stack1.push(value);
    }

    public T deleteHead(){
        T temp;
        if (stack2.size() <= 0){
            while(stack1.size() > 0){
                temp = (T)stack1.pop();
                stack2.push(temp);
            }
        }
        if (stack2.size() == 0){
            System.out.println("The Queue is empty!!");
        }
        return (T)stack2.pop();
    }
}

实现类：

package com.yyq;

/**
 * Created by Administrator on 2015/9/8.
 */
public class QueueWithTwoStacks {
    void Test(char actual, char expected)
    {
        if(actual == expected)
            System.out.println("Test passed.\n");
        else
            System.out.println("Test failed.\n");
    }

    public static void main(String args[]){
        QueueWithTwoStacks queueWithTwoStacks = new QueueWithTwoStacks();
        Queue<Character> queue = new Queue<Character>();
        queue.appendTail('a');
        queue.appendTail('b');
        queue.appendTail('c');
        queue.appendTail('d');
        queue.appendTail('e');
        queue.appendTail('f');

        char head = queue.deleteHead();
        queueWithTwoStacks.Test(head, 'a');

        head = queue.deleteHead();
        queueWithTwoStacks.Test(head, 'b');

        queue.appendTail('g');
        head = queue.deleteHead();
        queueWithTwoStacks.Test(head, 'c');

        queue.appendTail('h');
        head = queue.deleteHead();
        queueWithTwoStacks.Test(head, 'd');

        head = queue.deleteHead();
        queueWithTwoStacks.Test(head, 'e');
    }
}

 

输出结果：

Test passed.

 

Test passed.

 

Test passed.

 

Test passed.

 

Test passed.

 

Process finished with exit code 0