Implement Queue using Stacks 用俩栈实现队列-优快云博客

出处

http://www.programcreek.com/2014/07/leetcode-implement-queue-using-stacks-java/

http://blog.youkuaiyun.com/xudli/article/details/46784153

Implement the following operations of a queue using stacks.

push(x) -- Push element x to the back of queue.
pop() -- Removes the element from in front of queue.
peek() -- Get the front element.
empty() -- Return whether the queue is empty.

Notes:

You must use only standard operations of a stack -- which means onlypush to top,peek/pop from top,size, and is empty operations are valid.
Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

//第一种写法对于pop以及peek还有判断是否为空比较方便，都是直接在value上边判断的
//第二种虽然操作看起来比第一种貌似稍微麻烦一些，其实懂得了原理之后，会发现很不错，移动元素次数应该比第一种少
/*class MyQueue {
    // Push element x to the back of queue.
    Stack<Integer> temp=new Stack<Integer>();//临时中转的栈
    Stack<Integer> value=new Stack<Integer>();//真正保存数据的栈
    public void push(int x) {
        if(value.isEmpty())
            value.push(x);//第一个入栈的元素
        else{
            while(!value.isEmpty())
                temp.push(value.pop());//循环把value中的元素临时转移到temp

            value.push(x);//把新入的那个元素放入value栈底

            while(!temp.isEmpty())
                value.push(temp.pop());//再循环把上上步移到temp的元素转移回value
        }

    }//静心根据栈的后进先出特性，画一下流程就能很清楚的知道实现了队列的先进先出

    // Removes the element from in front of queue.
    public void pop() {
        value.pop();
    }

    // Get the front element.
    public int peek() {
        return value.peek();
    }

    // Return whether the queue is empty.
    public boolean empty() {
        return value.isEmpty();
    }
}*/

class MyQueue {//才发现在leetcode里边，不把这个class前边的空格删除是会报错的

    Stack<Integer> s1 = new Stack<>(); //s1是用来放新进来的元素的
    Stack<Integer> s2 = new Stack<>();

    // Push element x to the back of queue.
    public void push(int x) {
        s1.push(x);
    }

    // Removes the element from in front of queue.
    public void pop() {
        if(!s2.isEmpty()) s2.pop();
        else {
            while(!s1.isEmpty()) s2.push(s1.pop());
            s2.pop();
        }
    }

    // Get the front element.
    public int peek() {
        if(!s2.isEmpty()) return s2.peek();
        else {
            while(!s1.isEmpty()) s2.push(s1.pop());
            return s2.peek();
        }
    }

    // Return whether the queue is empty.
    public boolean empty() {
        return s1.empty() && s2.empty();
    }
}