[Leetcode] 989. Add to Array-Form of Integer

 Problem : https://leetcode.com/problems/add-to-array-form-of-integer/description/

Add digit on each position from back. 

Time complexity = O (max(len(num), log(k))

class Solution {
    public List<Integer> addToArrayForm(int[] num, int k) {
        LinkedList<Integer> result = new LinkedList<>();
        int carry = 0;
        int i = num.length - 1;

        while (i >= 0 || k > 0 || carry != 0) {
            int a = k % 10;
            int b = i >= 0 ? num[i--] : 0;

            k /= 10;

            result.addFirst((a + b + carry) % 10);
            carry = (a + b + carry) / 10;
        }

        return result;
    }
}