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.
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).

用堆栈实现队列的功能，这里我们借助两个堆栈来模拟队列的先进先出的特性。代码如下：

class MyQueue {
    Stack<Integer> stack1 = new Stack<Integer>();
    Stack<Integer> stack2 = new Stack<Integer>();
    // Push element x to the back of queue.
    public void push(int x) {
        stack1.push(x);
    }

    // Removes the element from in front of queue.
    public void pop() {
        while(stack1.size() > 1) {
            stack2.push(stack1.pop());
        }
        stack1.pop();
        while(!stack2.isEmpty())
            stack1.push(stack2.pop());
    }

    // Get the front element.
    public int peek() {
        while(stack1.size() > 1) {
            stack2.push(stack1.pop());
        }
        int result = stack1.peek();
         while(!stack2.isEmpty())
            stack1.push(stack2.pop());
        return result;
    }

    // Return whether the queue is empty.
    public boolean empty() {
        return stack1.isEmpty();
    }
}