public int sum(ListNode<Integer> n1, ListNode<Integer> n2) {
    validateInput(n1, n2);

    int n1Value = calculateNumber(n1);
    int n2Value = calculateNumber(n2);
    return n1Value + n2Value;
  }
Iterative solution to solve this problem. The complexity order is the same than in the
previous algorithm in time and space terms, but the execution time is bigger here because we
need to know the list size to be able to transform the list into an integer.
src/main/java/com/github/pedrovgs/problem61/SumLists.java