225.Implement Stack using Queues

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Stack, Easy

Question

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).

My Answer

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class MyStack {
    Queue<Integer> stack = new LinkedList<>();
    /** Initialize your data structure here. */
    public MyStack() {
        
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        stack.add(x);
        for(int i=1;i<stack.size();i++)
            stack.add(stack.poll());
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return stack.remove();
    }
    
    /** Get the top element. */
    public int top() {
        return stack.peek();
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return stack.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();
 */

Running time: 98ms