from typing import List

def plusOne(digits: List[int]) -> List[int]:
    i = len(digits) - 1
    digits[i] = digits[i] + 1
    while i > 0:
        if digits[i] == 10:
            digits[i - 1] = digits[i - 1] + 1
            digits[i] = 0
            i -= 1
        else:
            break

    if digits[0] == 10:
        digits[0] = 0
        digits.insert(0, 1)
    return digits

print(plusOne([9,9,9,0]))