232. Implement Queue using Stacks

mplement 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.

需要的就是两个栈来回倒腾

class MyQueue {
    private Stack<Integer> a=new Stack<Integer>();
    private Stack<Integer> b=new Stack<Integer>();
    // Push element x to the back of queue.
    public void push(int x) {
        a.push(x);
    }


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


    // Get the front element.
    public int peek() {//这里是整个的重点，取出元素
        if(b.empty())
        while(!a.empty())
        {
            b.push(a.pop());
        }
        return b.peek();
    }


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