Given a non-negative number represented as an array of digits, plus one to the number. The digits are stored such that the most significant digit is at the head of the list.

public class Solution {
    public int[] plusOne(int[] digits) {
        int len = digits.length;
        for (int i = len - 1; i >= 0; i--) {
            if (digits[i] < 9) {
                digits[i] ++;
                return digits;
                //if digits[i] is less than 9, we simply add 1
            } else {
               //if digit[i] is equal to 9, by adding 1, we make it 0, and since return is not executed at digits[i-1], digit[i-1] == 9
                digits[i] = 0;
            }
        }
        int[] newDigits = new int[len + 1];
        newDigits[0] = 1;
        return newDigits;
    }
}

class Solution(object):
    def plusOne(self, digits):
        """
        :type digits: List[int]
        :rtype: List[int]
        """

        if not digits:
            return []

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

        return [1] + [0] * len(digits)