public long getRecursive(long n) {
    validateInput(n);
    if (n == 1 || n == 2) {
      return 1;
    } else {
      return getRecursive(n - 1) + getRecursive(n - 2);
    }
  }
Recursive solution based on Fibonacci sequence definition. F(N) = F(N-1) + F(N-2).

The complexity order of this algorithm is O(2^N) where N is integer used as parameter. In space
terms, the complexity order of this algorithm is O(1) because we are not using any auxiliary
data structure to solve this problem.
src/main/java/com/github/pedrovgs/problem7/FibonacciNumbers.java