Package g0201_0300.s0225_implement_stack_using_queues

Class MyStack

  • public class MyStack
extends Object
    225 - Implement Stack using Queues.

    Easy

    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.

    Notes:

    • You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.
    • Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue’s standard operations.

    Example 1:

    Input [“MyStack”, “push”, “push”, “top”, “pop”, “empty”] [ [], [1], [2], [], [], []]

    Output: [null, null, null, 2, 2, false]

    Explanation:

     MyStack myStack = new MyStack();
 myStack.push(1);
 myStack.push(2);
 myStack.top(); // return 2
 myStack.pop(); // return 2
 myStack.empty(); // return False

    Constraints:

    • 1 <= x <= 9
    • At most 100 calls will be made to push, pop, top, and empty.
    • All the calls to pop and top are valid.

    Follow-up: Can you implement the stack using only one queue?

    • Constructor Detail

      • MyStack

        public MyStack()

    • Method Detail

      • push

        public void push​(int x)

      • pop

        public int pop()

      • top

        public int top()

      • empty

        public boolean empty()