chal_66.py

class Solution(object):
    def plus_one(self, digits):
        """
        :type digits: List[int]
        :rtype: List[int]
        """
        carry = 0
        index = 0
        added = False
        digits = digits[::-1]

        while not added or carry:
            if index < len(digits):
                if digits[index] == 9:
                    digits[index] = 0
                    carry = 1
                else:
                    digits[index] += 1
                    carry = 0
                added = True
            else:
                digits.append(carry)
                carry = 0
            index += 1
        return digits[::-1]


# Test cases
sol = Solution()

digits = [1, 2, 3]
assert sol.plus_one(digits) == [1, 2, 4]

digits = [4, 3, 2, 1]
assert sol.plus_one(digits) == [4, 3, 2, 2]

digits = [9]
assert sol.plus_one(digits) == [1, 0]