addToArrayForm.js created by smakar20 - https://repl.it/@smakar20/addToArrayFormjs

index.js

/* 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
 

Note：

1 <= A.length <= 10000
0 <= A[i] <= 9
0 <= K <= 10000
If A.length > 1, then A[0] != 0
 */

var addToArrayForm = function(A, K) {
  var carry = 0;
  var output = [];
  var toAdd = String(K).split('');
  var iterator = ((A.length > toAdd.length) ? A : toAdd).reverse();
  var toAddNum = ((A.length > toAdd.length) ? toAdd : A).reverse();
  for (var i = 0; i < iterator.length; i++){
    var sum = Number(iterator[i]) + carry;
    if (toAddNum[i] != undefined) sum += Number(toAddNum[i]);

    carry = 0;
    if (sum > 9){
      var strSum = String(sum);
      carry = Number(strSum.substr(0, strSum.length - 1));
      output.push(strSum.substr(strSum.length - 1));
      continue;
    }
    output.push(sum);
  }
  if (carry !== 0) output.push(carry);
  return output.reverse(); 
};

addToArrayForm([2,1,5],
806); // [1, 0, 2, 1]