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.
-------------------------------------------------------------------------------
*/