Package g0201_0300.s0232_implement_queue_using_stacks

Class MyQueue

  • All Implemented Interfaces:
    
public final class MyQueue

    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.

    • Nested Class Summary

      Nested Classes 
      Modifier and Type Class Description

    • Field Summary

      Fields 
      Modifier and Type Field Description

    • Constructor Summary

      Constructors 
      Constructor Description
      MyQueue()

    • Enum Constant Summary

      Enum Constants 
      Enum Constant Description

    • Method Summary

      Modifier and Type Method Description
      final Unit push(Integer x)
      final Integer pop()
      final Integer peek()
      final Boolean empty()

      • Methods inherited from class java.lang.Object

        clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait