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.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front

  , size, and is emptyoperations 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).

Analysis

https://leetcode.com/articles/implement-stack-using-queues/

Solution

One Queue Implementation

class MyStack {
    private Queue<Integer> queue;
    /** Initialize your data structure here. */
    public MyStack() {
        queue = new LinkedList<Integer> (); 
    }

    /** Push element x onto stack. */
    public void push(int x) {
        queue.add(x);
        for (int i = 1; i < queue.size(); i++) {
            queue.add(queue.poll());
        }
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
       return queue.poll(); 
    }

    /** Get the top element. */
    public int top() {
       return queue.peek(); 
    }

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

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * boolean param_4 = obj.empty();
 */