public List<BinaryNode> getRecursive(BinaryNode root) {
    validateTree(root);
    return getInner(root);
  }
Recursive implementation of this binary tree traversal. The complexity order of this
algorithms in time terms is O(N) and O(N) in space terms because we are using one additional
data structure to return the result.
src/main/java/com/github/pedrovgs/problem16/BinaryTreePostOrder.java