def increment(digits):

    for i in range(len(digits) - 1, -1, -1):
        if digits[i] < 9:
            digits[i] += 1
            return digits
        digits[i] = 0
    
    return [1] + digits

if __name__ == "__main__":

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

    print(increment([9, 9, 9])) 
   
    print(increment([4, 3, 2, 1])) 

    print(increment([1, 9, 9, 9]))

    print(increment([9]))