989. Add to Array-Form of Integer

/*
For a non-negative integer X, 
the array-form of X is an array of its digits in left to right order.  
For example, if X = 1231, then the array form is [1,2,3,1].

Given the array-form A of a non-negative integer X, return the array-form of the integer X+K.

Example 1:
Input: A = [1,2,0,0], K = 34
Output: [1,2,3,4]
Explanation: 1200 + 34 = 1234

Example 2:
Input: A = [2,7,4], K = 181
Output: [4,5,5]
Explanation: 274 + 181 = 455

Example 3:
Input: A = [2,1,5], K = 806
Output: [1,0,2,1]
Explanation: 215 + 806 = 1021

Example 4:
Input: A = [9,9,9,9,9,9,9,9,9,9], K = 1
Output: [1,0,0,0,0,0,0,0,0,0,0]
Explanation: 9999999999 + 1 = 10000000000
*/


class Solution {
    public List<Integer> addToArrayForm(int[] A, int K) {
        List<Integer> result = new ArrayList<>();
        int digit;
        int X = 0;

        for (int i = 0; i < A.length; i++) {
            digit = A[i];
            X = X * 10 + digit;
        }
        
        X = X + K;
        
        do {
            result.add(0, X % 10);
            X /= 10;
        } while  (X > 0);
        
        return result;
    }
}