private int getDepth(BinaryNode root) {
    if (root == null) {
      return 0;
    } else {
      return 1 + Math.max(getDepth(root.getLeft()), getDepth(root.getRight()));
    }
  }
Calculate the Binary Tree depth based on recursion. The complexity order in space terms of
this algorithm is O(N) and in time terms is O(N) where N is the number of nodes in the tree.
src/main/java/com/github/pedrovgs/problem13/BinaryTreeByLevel.java