def _bstSearch( self, subtree, target ):        
    """Helper method that recursively searches the tree for a target key."""
    if subtree is None :         # base case, not found
      return None
    elif target < subtree.key :  # target is left of the subtree root.
      return self._bstSearch( subtree.left, target )
    elif target > subtree.key :  # target is right of the subtree root.
      return self._bstSearch( subtree.right,target )       
    else :                       # base case, found!
      return subtree