Determing if a binary tree is balanced

main.cpp

// Note that this is an interesting kind of tree, where there is no overall structure that contains nodes.
// Instead the tree is itself a node.
template<class ItemType>
class Tree {
 private:
   Tree<ItemType> * left;
   Tree<ItemType> * right;
   ItemType data;   
   
 public: 
   Tree(ItemType _data){left = NULL; right = NULL; data = _data;}
   void insert(ItemType _data){
      if ( _data > data){
         if (right == NULL){
            right = new Tree(_data);
         } else{
            right->insert(_data);
         }
      } else{
         if (left == NULL){
            left = new Tree(_data);
         } else{
            left->insert(_data);
         }
      }
   }
      
   bool isBalanced(){
      if(left == NULL && right == NULL) return true; 
      if(left == NULL && right != NULL) return right->height() == 0; 
      if(right == NULL && left != NULL) return left->height() == 0; 
      return left->isBalanced() && right->isBalanced() && abs(left->height() - right->height()) <= 1;
   }
   
   int height(){
      if(left == NULL && right == NULL) return 0;
      if(left == NULL && right != NULL) return right->height() + 1;
      if(right == NULL && left != NULL) return left->height() + 1;
      return max(right->height(), left->height()) + 1;
   }
};