Given an array digits that represents a non-negative integer, add one to the number and return the result as an array.

AddOne.kt

/*
Given an array digits that represents a non-negative integer, add one to the number and return the result as an array. 
Ex: Given the following digits...

digits = [1, 2], return [1, 3].
Ex: Given the following digits...

digits = [9, 9], return [1, 0, 0].
* */
fun plusOne(digits: IntArray): IntArray {
    for (i: Int in digits.lastIndex downTo 0) {
        if (digits[i] != 9) {
            digits[i]++
            return digits
        } else {
            digits[i] = 0
        }
    }
    val result = IntArray(digits.size + 1)
    result[0] = 1
    return result
}