LeetCode in Kotlin

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?

Solution

import java.util.LinkedList

class MyStack {
    private var queuePair = Pair(LinkedList<Int>(), LinkedList<Int>())
    private var top: Int? = null

    fun push(x: Int) {
        queuePair.first.addLast(x)
        top = x
    }

    fun pop(): Int {
        if (isQueuesEmpty()) {
            throw Exception()
        }
        val queuePair = selectSourceAndDestinationQueues(queuePair)
        var value = 0
        repeat(queuePair.first.size) {
            when (queuePair.first.size) {
                2 -> {
                    top = queuePair.first.removeFirst()
                    queuePair.second.addLast(top)
                }
                1 -> {
                    value = queuePair.first.removeFirst()
                }
                else -> {
                    queuePair.second.addLast(queuePair.first.removeFirst())
                }
            }
        }
        return value
    }

    fun top(): Int {
        if (isQueuesEmpty()) {
            throw Exception()
        }
        return top!!
    }

    fun empty(): Boolean {
        return isQueuesEmpty()
    }

    private fun isQueuesEmpty(): Boolean {
        if (queuePair.first.isEmpty() && queuePair.second.isEmpty()) {
            return true
        }
        return false
    }

    private fun selectSourceAndDestinationQueues(queuePair: Pair<LinkedList<Int>, LinkedList<Int>>):
        Pair<LinkedList<Int>, LinkedList<Int>> {
        return if (queuePair.first.isNotEmpty()) {
            Pair(queuePair.first, queuePair.second)
        } else {
            Pair(queuePair.second, queuePair.first)
        }
    }
}

/*
 * Your MyStack object will be instantiated and called as such:
 * var obj = MyStack()
 * obj.push(x)
 * var param_2 = obj.pop()
 * var param_3 = obj.top()
 * var param_4 = obj.empty()
 */

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