Package g0201_0300.s0232_implement_queue_using_stacks

Class MyQueue

  • public class MyQueue
extends Object
    232 - Implement Queue using Stacks.

    Easy

    Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).

    Implement the MyQueue class:

    • void push(int x) Pushes element x to the back of the queue.
    • int pop() Removes the element from the front of the queue and returns it.
    • int peek() Returns the element at the front of the queue.
    • boolean empty() Returns true if the queue is empty, false otherwise.

    Notes:

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

    Example 1:

    Input:

     ["MyQueue", "push", "push", "peek", "pop", "empty"]
 [ [], [1], [2], [], [], []]

    Output:

     [null, null, null, 1, 1, false]

    Explanation:

     MyQueue myQueue = new MyQueue();
 myQueue.push(1); // queue is: [1]
 myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
 myQueue.peek(); // return 1
 myQueue.pop(); // return 1, queue is [2]
 myQueue.empty(); // return false

    Constraints:

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

    Follow-up: Can you implement the queue such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.

    • Constructor Detail

      • MyQueue

        public MyQueue()

    • Method Detail

      • push

        public void push​(int x)

      • pop

        public int pop()

      • peek

        public int peek()

      • empty

        public boolean empty()