Leetcode 232. Implement Queue using Stacks

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.

题意：用栈来实现队列。

思路：用一个栈来存放push进的元素，另一个栈存放要pop出的元素。当调用pop或peek时，如果pop栈是空的，就把push栈里的元素从栈顶一个个取出来push进pop栈，这样pop栈的栈顶就是最早push进来的元素。

class MyQueue {

    private Stack pushStack;
    private Stack popStack;

    /** Initialize your data structure here. */
    public MyQueue() {
        this.pushStack = new Stack<>();
        this.popStack = new Stack<>();
    }

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

    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        this.reloadPop();
        return this.popStack.pop();
    }

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

    /** Returns whether the queue is empty. */
    public boolean empty() {
        return this.pushStack.empty() && this.popStack.empty();
    }

    private void reloadPop() {
        if (this.popStack.size() == 0) {
            while (this.pushStack.size() > 0) {
                this.popStack.push(this.pushStack.pop());
            }
        }
    }
}