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.

Notes:

• You must use only standard operations of a stack -- which means only push to top, peek/pop from top, size, and is emptyoperations 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).

题目含义：使用stack来实现queue的功能

class MyQueue {
    Stack<Integer> input = new Stack();
    Stack<Integer> output = new Stack();
    /** Initialize your data structure here. */
    public MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        input.push(x);     
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
             peek();
        return output.pop();   
    }
    
    /** Get the front element. */
    public int peek() {
        if (output.empty())
            while (!input.empty())
                output.push(input.pop());
        return output.peek();
        
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
         return input.empty() && output.empty();
    }
}