225. 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 backpeek/pop from frontsize, 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).
class MyStack {
    
    Queue<Integer> q1 = new LinkedList<Integer>();
    Queue<Integer> q2 = new LinkedList<Integer>();
    boolean isone = true;

    /** Initialize your data structure here. */
    public MyStack() {
        
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        if(isone){
            q1.add(x);
        }
        else{
            q2.add(x);
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        if(isone){
            isone = false;
            while(q1.size() != 1){
                q2.add(q1.poll());
            }
            return q1.poll();
            
        }
        else{
            isone = true;
            while(q2.size() != 1){
                q1.add(q2.poll());
            }
            return q2.poll();            
        }
    }
    
    /** Get the top element. */
    public int top() {
         if(isone){
            isone = false;
            while(q1.size() != 1){
                q2.add(q1.poll());
            }
            int res = q1.poll();
            q2.add(res);
            return res;
            
        }
        else{
            isone = true;
            while(q2.size() != 1){
                q1.add(q2.poll());
            }
            int res = q2.poll();
            q1.add(res);
            return res;            
        }       
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        if(isone){
            return q1.isEmpty();
        }
        else{
            return q2.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();
 */

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