Implement Stack using Queues

Source

• leetcode: 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).

Java

class MyStack {
    // Push element x onto stack.
    Queue<Integer> queue = new LinkedList<Integer>();

    public void push(int x) {
        queue.add(x);
    }

    // Removes the element on top of the stack.
    public void pop() {
        for(int i=1; i<queue.size(); i++){
            queue.add(queue.remove());
        }
        queue.remove();
    }

    // Get the top element.
    public int top() {
        for(int i=1; i<queue.size(); i++){
            queue.add(queue.remove());
        }
        int tmp = queue.remove();
        queue.add(tmp);
        return tmp;
    }

    // Return whether the stack is empty.
    public boolean empty() {
        return queue.isEmpty();
    }
}