Leetcode #66: "Given a non-empty array of digits representing a non-negative integer, plus one to the integer. The digits are stored such that the most significant digit is at the head of the list, and each element in the array contain a single digit. You may assume the integer does not contain any leading zero, except the number 0 itself."

PlusOne.swift

class Solution {
    func plusOne(_ digits: [Int]) -> [Int] {
        guard !digits.isEmpty else {
            fatalError("But you promised!")
        }
        var digits = digits
        let lastIndex = digits.count - 1
        for i in 0...lastIndex {
            let index = lastIndex - i
            var digit = digits[index]
            if digit != 9 {
                digit += 1
                digits[index] = digit
                break
            }
            else {
                digits[index] = 0
                if index == 0 {
                    digits.insert(1, at: 0)
                }
            }
        }
        return digits
    }
}

/*
Leetcode results:
------------------------------------------------------------------------------
Runtime: 8 ms, faster than 78.25% of Swift online submissions for Plus One.
Memory Usage: 21 MB, less than 20.00% of Swift online submissions for Plus One.
-------------------------------------------------------------------------------
*/