284. Peeking Iterator

package LeetCode_284

/**
 * 284. Peeking Iterator
 * https://leetcode.com/problems/peeking-iterator/
 * Design an iterator that supports the peek operation on a list in addition to the hasNext and the next operations.
Implement the PeekingIterator class:
PeekingIterator(int[] nums): Initializes the object with the given integer array nums.
int next(): Returns the next element in the array and moves the pointer to the next element.
bool hasNext(): Returns true if there are still elements in the array.
int peek(): Returns the next element in the array without moving the pointer.
 * */
// Kotlin Iterator reference:
// https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/-iterator/

class PeekingIterator(iterator: Iterator<Int>) : Iterator<Int> {

    private var isDone = false
    private var it: Iterator<Int>? = null
    private var next: Int = 0

    init {
        it = iterator
        if (iterator.hasNext()){
            next = iterator.next()
        } else {
            isDone = true
        }
    }

    fun peek(): Int {
        return next
    }

    override fun next(): Int {
        //return value and move the pointer
        val result = next
        if (it?.hasNext()!!) {
            next = it?.next()!!
        } else {
            isDone = true
        }
        return result
    }

    override fun hasNext(): Boolean {
        return !isDone
    }
}

/**
 * Your PeekingIterator object will be instantiated and called as such:
 * var obj = PeekingIterator(arr)
 * var param_1 = obj.next()
 * var param_2 = obj.peek()
 * var param_3 = obj.hasNext()
 */