Question

In the chapter on binary search tree’s in CLRS and I encountered the transplant function which replaces a node u with node v with appropriate changes in the parent element.
Here is the code I wrote for transplant function:

void transplant(Node* root, Node* u, Node* v)
{
    if(u->parent == NULL)
        root = v;
    else if(u == u->parent->left)
        u->parent->left = v;
    else
        u->parent->right = v;
    if(v != NULL)
        v->parent = u->parent;
}

It’s not that I don’t understand how this works but that why this works. When I make a function call I’m basically sending a copy of pointers root, u, v to the function right ? so the changes made in the function shouldn’t actually reflect on the root unless I return it or use pointer to pointer type but its actually changing the original root. I defined root as global variable, does that change anything ?

Answer 1

The function will not work if u->parent is NULL, in that case, only the local variable root is set to v, nothing accessible from outside the function changes.

If u->parent != NULL, then u->parent->left and u->parent->right are the members of the Node pointed to by u->parent, and those are overwritten, so the changes are visible in main.