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).

可以在push阶段利用两个栈实现栈中元素的反转。

class MyQueue {
	Stack<Integer> value=new Stack<Integer>();
	Stack<Integer> tmp=new Stack<Integer>();
    // Push element x to the back of queue.
    public void push(int x) {
    	if(value.isEmpty())
    		value.push(x);
    	else{
    		while(!value.isEmpty())
    			tmp.push(value.pop());
    		value.push(x);
    		while(!tmp.isEmpty())
    			value.push(tmp.pop());
    	}
    }
    // Removes the element from in front of queue.
    public void pop() {
    	value.pop();
    }
    // Get the front element.
    public int peek() {
        return value.peek();
    }
    // Return whether the queue is empty.
    public boolean empty() {
        return value.isEmpty();
    }
}