Coding Exercises Day 12 (Problem #16)

Problem:
You have two numbers represented by a linked list, where each node contains a single digit.  The digits are stored in reverse order, such that the 1's digit is at the head of the list.  Write a function that adds the two numbers and returns the sum as a linked list.

Example:
Input: (7 -> 1 -> 6) + (5 -> 9 -> 2).  Translates to 617 + 295.
Output: 2 -> 1 -> 9.  Translates to 912.

Follow Up:
Suppose the digits are stored in forward order.  Repeat the above problem.

Example:
Input: (6 -> 1 -> 7) + (2 -> 9 -> 5).  Translates to 617 + 295.
Output: 9 -> 1 -> 2.  Translates to 912.

List sum(List<Integer> numOneList, List<Integer> numTwoList, boolean isReversed) {
  StringBuilder numOneString = new StringBuilder();
  StringBuilder numTwoString = new StringBuilder();
   
  for (Integer digit : numOneList) {
    numOneString.append(digit);
  }

  for (Integer digit : numTwoList) {
    numOneString.append(digit);
  }

  if (isReversed) {
    numOneString.reverse();
    numTwoString.reverse();
  }

  int sum = Integer.parseInt(numOneString.toString()) 
         + Integer.parseInt(numTwoString.toString());

  List<Integer> rval = new LinkedList<Integer>();
  
  String sumString = String.valueOf(sum);

  for (int i = 0; i < sumString.length(); i++) {
    int j = i;
    if (isReversed) {
      j = sumString.length() - 1 - i;  
    }
    rval.add(sumString.charAt(j));
  }
  return rval;
}