public int get(BinaryNode root) {
    if (root == null) {
      throw new IllegalArgumentException("You can't pass a null BinaryNode as parameter");
    }
    return getInner(root);
  }
Recursive solution for this problem. The complexity order of this algorithm in time terms is
O(N) where N is the number of elements in the binary tree. In space terms, the complexity order
of this algorithm is O(1) because we are not using any additional data structure related to the
number of nodes in the tree to resolve this problem.
src/main/java/com/github/pedrovgs/problem19/BinaryTreeDepth.java