989. Add to Array-Form of Integer

// LeetCode URL: https://leetcode.com/problems/add-to-array-form-of-integer/
import java.util.LinkedList;
import java.util.List;

/**
 * Iterate over the input array from end to start and keep adding K.
 *
 * Time Complexity: O(N + K)
 *
 * Space Complexity: O(1)
 *
 * N = Length of A. K = Number of digits in K
 */
class Solution {
    public List<Integer> addToArrayForm(int[] A, int K) {
        LinkedList<Integer> result = new LinkedList<>();
        if (A == null) {
            throw new IllegalArgumentException("Input array A is null");
        }

        for (int i = A.length - 1; i >= 0; i--) {
            int sum = A[i] + K;
            result.addFirst(sum % 10);
            K = sum / 10;
        }

        while (K > 0) {
            result.addFirst(K % 10);
            K /= 10;
        }

        return result;
    }
}