BST insert value - c++

Question

I'm having a hard time to get the insert function to work correctly. The cout statement isn't matching the one for my assignment. If I try to insert 5, I get inserted: 5 right child of: 3 but it should be inserted: 5 right child of: 22 Can someone help me out?
Node* insert(Node *root, int value){
Node *tmp = root;
while(tmp != NULL){
if (tmp->key == value){
return tmp;
}
else if (value > tmp->key){
Node *tm = new Node(NULL, NULL, NULL);
tm->key = value;
tm->left = tmp->right;
tm->right = tmp->left;
tmp->right = tm;
cout << "inserted: " << tm->key << " right child of: " << tmp->key <<endl;
tmp = tmp->right;
}
else if (value < tmp->key){
Node *tm = new Node(NULL, NULL, NULL);
tm->key = value;
tm->left = NULL;
tm->right = NULL;
tmp->left = tm;
cout << "inserted: " << tm->key << " left child of: " << tmp->key <<endl;
tmp = tmp->left;
}
}
return tmp;
}

Answer 1

There are a few problems with your insertion code:
You always insert the new node as a child of the root (which does not always produce a BST)
When you link your tree, you either (if you place as right child) make the left child of the root also the left child of the new node, or (if left child) lose track of the original left subtree of root.
What happens if root is NULL
The following code should work for you
//The following code assumes the following definition of Node
struct Node {
Node(int val, Node* l, Node* r):key(val),left(l),right(r){}
int key;
Node* left;
Node* right;
}
// Returns the root of the tree after inserting val into tree (no duplicates)
Node * insert (Node * root, int val){
if (root == NULL) return new Node(val, NULL, NULL);
if (root->key == val) return root;
if (root->key > val) return insert(root->left, val);
return insert(root->right, val);
}
The above code defines insert recursively. Base case : tree is empty. Insert new value as the root. If the root is the key then you're done (no duplicated keys). Otherwise, insert the value into the left or right subtree (left if value is less than root->key or right otherwise).
Or you can define insert iteratively using a while loop.
Node * insert (Node * root, int val){
// is this an empty tree?
if (root == NULL) return new Node(val, NULL, NULL);
Node* tmp = root;
while (tmp != null){
if (tmp->key == val) break;
if (tmp->key > val){
if (tmp->left == NULL){
tmp->left = new Node(val, NULL, NULL);
break;
} else {
tmp = tmp->left;
}
} else {
if (tmp->right == NULL){
tmp->right = new Node(val, NULL, NULL);
break;
} else {
tmp = tmp->right;
}
}
}
return root;
}
In either case you can use insert the same way.
Node * tree1 = NULL;
tree1 = insert(tree1, 3);
tree1 = insert(tree1, 1);
tree1 = insert(tree1, 2);
tree1 = insert(tree1, 4);
Aftewards tree1 will be the following tree.
3
/ \
1 4
\
2