Your code is mixing two common approaches to the task, hence the problem. You are also using an abstract data type (ADT) type approach, rather than an object-oriented one, so there are three approaches to consider.
In both ADT approaches your tree is represented by a reference to its root, in Objective-C this is probably stored in an instance variable:
Node *TreeRoot;
Note also that both of these algorithms use field references, a->b, rather than property references, a.b - this is because the former references a variable and the second algorithm requires passing a reference to a variable.
Functional ADT: Pass-by-value and assign result
In this approach a node is inserted into a tree and a modified tree is returned which is assigned back, e.g. the top-level call to insert a Node nodeToInsert would be:
TreeRoot = insertNode(nodeToInsert, TreeRoot);
and the insertNode function looks like:
Node *insertNode(Node *node, Node *root)
{
if(root == nil)
{ // empty tree - return the insert node
return node;
}
else
{ // non-empty tree, insert into left or right subtree
if(node->data > root->data) // to the right
{
root->right = insertNode(node, root->right);
}
else if(node->data < root->data)//or to the left
{
root->left = insertNode(node, root->left);
}
// tree modified if needed, return the root
return root;
}
}
Note that in this approach in the case of a non-empty (sub)tree the algorithm performs a redundant assignment into a variable - the assigned value is what is already in the variable... Because of this some people prefer:
Procedural ADT: Pass-by-reference
In this approach the variable holding the root of the (sub)tree is passed-by-reference, rather than its value being passed, and is modified by the called procedure as needed. E.g. the top-level call would be:
insertNode(nodeToInsert, &TreeRoot); // & -> pass the variable, not its value
and the insertNode procedure looks like:
void insertNode(Node *node, Node **root)
{
if(*root == nil)
{ // empty tree - insert node
*root = node;
}
else
{ // non-empty tree, insert into left or right subtree
Node *rootNode = *root;
if(node->data > rootNode->data) // to the right
{
insertNode(node, &rootNode->right);
}
else if(node->data < rootNode->data)//or to the left
{
insertNode(node, &root->left);
}
}
}
You can now see that your method is a mixture of the above two approaches. Both are valid, but as you are using Objective-C it might be better to take the third approach:
Object-Oriented ADT
This is a variation of the procedural ADT - rather than pass a variable to a procedure the variable, now called an object, owns a method which updates itself. Doing it this way means you must test for an empty (sub)tree before you make a call to insert a node, while the previous two approaches test in the call. So now we have the method in Node:
- (void) insert:(Node *)node
{
if(node.data > self.data) // using properties, could also use fields ->
{
if(self.right != nil)
[self.right insert:node];
else
self.right = node;
}
else if(node.data < rootNode.data)
{
if(self.left != nil)
[self.left insert:node];
else
self.left = node;
}
}
You also need to change the top level call to do the same test for an empty tree:
if(TreeRoot != nil)
[TreeRoot insert:nodeToInsert];
else
TreeRoot = nodeToInsert;
And a final note - if you are using MRC, rather than ARC or GC, for memory management you'll need to insert the appropriate retain/release calls.
Hope that helps you sort things out.
