Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

Update (2015-06-11):

The class name of the Java function had been updated to MyStack instead of Stack.

Credits:

Special thanks to for adding this problem and all test cases.

 Solution:

O(1)

class MyStack {    private Queue queue;        // Push element x onto stack.    public void push(int x) {        Queue q = new LinkedList();        q.add(x);        q.add(queue);        queue = q;                }    // Removes the element on top of the stack.    public void pop() {        queue.remove();        queue = (Queue) queue.peek();    }    // Get the top element.    public int top() {        return (int)queue.peek();    }    // Return whether the stack is empty.    public boolean empty() {        return queue==null;    }}