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 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).

解题：

用栈实现队列，这个比用队列实现栈要麻烦一些，这里用到两个栈，两种思路。第一种就是在进栈的时候，把栈逆序放在另外一个栈，出的时候直接出另外一个栈就能够了。另外一种思路。进栈的时候不做不论什么处理。出栈的时候把栈逆序放在另外一个栈。出另外一个栈。以下就是两种的代码实现。

在进栈的时候进行处理：

class MyQueue {

    // Push element x to the back of queue.

	Stack<Integer> stack=new Stack<>();

	Stack<Integer> stack2=new Stack<>();

    public void push(int x) {

    	while(!stack.isEmpty())

    	{

    		stack2.push(stack.pop());

    	}

    	stack2.push(x);

    	while(!stack2.isEmpty())

    	{

    		stack.push(stack2.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();

    }

}

在出栈的时候进行处理：

class MyQueue2 {

    // Push element x to the back of queue.

	Stack<Integer> stack=new Stack<>();

	Stack<Integer> stack2=new Stack<>();

    public void push(int x) {

    	while(!stack2.isEmpty())

    		stack.push(stack2.pop());

    	stack.push(x);

    }

    // Removes the element from in front of queue.

    public void pop() {

    	while(!stack.isEmpty())

    		stack2.push(stack.pop());

        stack2.pop();

    }

    // Get the front element.

    public int peek() {

    	while(!stack.isEmpty())

    		stack2.push(stack.pop());

        return stack2.peek();

    }

    // Return whether the queue is empty.

    public boolean empty() {

    	while(!stack2.isEmpty())

    		stack.push(stack2.pop());

        return stack.isEmpty();

    }

}