232.Implement Queue using Stacks(用栈实现队列)

题目：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)

代码：

class MyQueue {
    // Push element x to the back of queue.
    Stack <Integer> stack = new Stack();
    Stack <Integer> tmp = new Stack();
    
    public void push(int x) {
        while(!stack.isEmpty()){
            tmp.push(stack.pop());
        }
        stack.push(x);
        while(!tmp.isEmpty()){
            stack.push(tmp.pop());
        }
    }


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


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


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