Implement Stack using Queues

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 @jianchao.li.fighter for adding this problem and all test cases.

Show Tags

Show Similar Problems

Have you met this question in a real interview?

思路： Every time after we add a new element, we need to pop all element and move them to tail.

class MyStack {    Queue<Integer> q = new LinkedList<Integer>();    // Push element x onto stack.    public void push(int x) {        q.add(x);        int n = q.size();        while(n > 1){            q.add(q.poll());            n--;        }    }    // Removes the element on top of the stack.    public void pop() {        q.poll();    }    // Get the top element.    public int top() {        return q.peek();    }    // Return whether the stack is empty.    public boolean empty() {        return q.isEmpty();    }}