def plusOne(digits):
    zeros = 0
    if digits[-1] == 9:
        if len(digits) < 2:
            digits[-1] = 1
            digits.append(0)
            return digits
        for i in range(len(digits)):
            if digits[i] == 9:
                zeros += 1
                if digits[i-1] == 9:
                    digits[i] = 0
                else:
                    digits[i] = 1
                    break
        digits.append(0)
    else:
        digits[-1] = digits[-1] + 1
    return digits


print(plusOne([1,2,3]))