public int getIterative(int n) {
    validateInput(n);

    int result = 1;
    for (int i = n; i >= 2; i--) {
      result *= i;
    }
    return result;
  }
Iterative solution for this problem. This version is based on the factorial definition
described in the statement and the complexity order in time terms of this solution is O(N)
where N is integer passed as argument. In space terms, the complexity order of this algorithm
is O(1) because we are not using any additional data structure related to the integer passed
as parameter.
src/main/java/com/github/pedrovgs/problem9/Factorial.java