Exception: Deletion of node in Binary Search Tree

Question

I am getting Exception when running the BST Deletion. Below is my code snippet:

Bst::node * Bst::del(node *root, int num)
    {
        if (root == NULL)
        {
            return root;
        }
        else if (num < root->data)
        {
            root->left = del(root->left, num);
        }
        else if (num > root->data)
        {
            root->right = del(root->right, num);
        }
        else
        {
            if (root->left == NULL)
            {
                node * tmp = root;
                root = root->right;
                delete tmp;
            }
            else if (root->right == NULL)
            {
                node * tmp = root;
                root = root->left;
                delete tmp;
            }
            else if (root->left == NULL && root->right == NULL)
            {
                delete root;
                root = NULL;
            }
            else
            {
                node *tmp = root;
                tmp = findMin(root->right);
                root->data = tmp->data;
                root->right = del(root->right, tmp->data);
            }
        }


        return root;
    }

---

void Bst::del(int num)
{
     del(root, num);
}

Everything works fine when I am deleting the other nodes but when I delete the root node itself then the function void Bst::del(int num) gets the garbage value from the function Bst::node * Bst::del(node *root, int num) . The error gets resolved when I rewrite my function as

void Bst::del(int num)
        {
            root = del(root, num);
        }

Question 1. Why it works when I delete the middle nodes or any other node except the root node. While debugging I found that even root was getting deleted properly when the function Bst::node * Bst::del(node *root, int num) was executing but when the call returned to the void Bst::del(int num) then the value of root was not getting retained and was garbage.

Question 2: Why the error got fixed when I stored the returned value in variable root ?

Answer 1

While deleting a BST node using recursion, you must track the root node, which you're doing correctly as

root->left = // ... and root->right = ...

However when call reaches to caller after unwinding the stack, the root may get modified ( case when you delete the root itself )

_{This hopefully answers both of your questions}