LeetCode 225. Implement Stack using Queues

Question

• Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

  Implement the MyStack class:

  • void push(int x) Pushes element x to the top of the stack.
  • int pop() Removes the element on the top of the stack and returns it.
  • int top() Returns the element on the top of the stack.
  • boolean empty() Returns true if the stack is empty, false otherwise.

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

class MyStack {
    //push O(n)
    public MyStack() {
    }
    Queue<Integer> q1 = new LinkedList<>();
    Queue<Integer> q2 = new LinkedList<>();
    public void push(int x){
        if(q1.isEmpty()) q1.offer(x);
        while(!q2.isEmpty()) q1.offer(q2.poll());
        while(!q1.isEmpty()) q2.offer(q1.poll());
    }
    public int pop(){return q2.poll();}
    public int top(){return q2.peek();}
    public boolean empty(){return q1.isEmpty() && 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();
 */

Complexity:

Time complexity:

push: O(n)

pop/top/empty: O(1)

Space complexity: O(n)