LeetCode 232. Implement Queue using Stacks 用栈实现队列

Question:

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 only push 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).

原题链接： https://leetcode.com/problems/implement-queue-using-stacks/description/

Stack类

实例化：Stack <E> 自定义栈名 = new Stack <E> ();

如： Stack <Integer> data = new Stack <Integer> ();

序号	方法描述
1	boolean empty() 测试堆栈是否为空。
2	Object peek( ) 查看堆栈顶部的对象，但不从堆栈中移除它。
3	Object pop( ) 移除堆栈顶部的对象，并作为此函数的值返回该对象。
4	Object push(Object element) 把项压入堆栈顶部。
5	int search(Object element) 返回对象在堆栈中的位置，以 1 为基数。

class MyQueue {

    /** Initialize your data structure here. */
    public MyQueue() {

    }
   private Stack <Integer> data = new Stack <Integer> ();
   private Stack <Integer> tmp = new Stack <Integer> ();
    /** Push element x to the back of queue. */
    public void push(int x) {
       // if(data.empty())
       //     data.push(x);
        while(data.empty()!=true) //empty()返回boolean
        {
            tmp.push(data.peek());
            data.pop();
        }
        tmp.push(x);
        while(tmp.empty()!=true)
        {
            data.push(tmp.peek());
            tmp.pop();
        }
    }

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        int front = data.peek();
        data.pop();
        return front;
    }

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

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return data.empty();
    }
}

/**
* Your MyQueue object will be instantiated and called as such:
* MyQueue obj = new MyQueue();
* obj.push(x);
* int param_2 = obj.pop();
* int param_3 = obj.peek();
* boolean param_4 = obj.empty();
*/

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LeetCode 232. Implement Queue using Stacks 用栈实现队列

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