Repair tree structure in c - c

Question

I have a tree structure in c (quadtree) which I want to clean to avoid long tree branches. Here is an example:
Instead of this tree structure,
I want to clean up the tree to get this structure (Same information, less memory consumption, faster tree walks).
Here the red node is the root of the tree. Blue nodes are empty and black nodes are tree nodes which carry information.
My tree has the following structure in c:
typedef enum{particle, pseudo_particle} node_type;
typedef struct{
double m;
double x[DIM];
int to_delete;
} Particle;
typedef struct{
double x_min[DIM];
double x_max[DIM];
} tree_box;
typedef struct tree_node{
Particle p;
tree_box box;
node_type node;
struct tree_node *child[4];
} tree_node;
The function which cleans up the tree
void clean_tree(tree_node *t){
if(t != NULL){
for(int i=0; i<4; i++){
clean_tree(t->child[i]);
}
// Operation on *t
if(t->node != particle){
int child_counter = 0;
int d = 0;
for(int i=0; i<4; i++) {
if(t->child[i] != NULL) {
if(t->child[i]->p.to_delete == TRUE){
free(t->child[i]);
t->child[i] = NULL;
}
else {
child_counter++;
d=i;
}
}
}
if(child_counter == 0) {
t->p.to_delete = TRUE;
}else if (child_counter == 1) {
t = t->child[d];
free(t->child[d]);
t->child[d] = NULL;
}
}
// End of operation on *t
}
}
But here is the problem: After calling clean_tree() my tree structure is no longer existent. The whole tree gets deleted. But why? Does someone see my error?

Answer 1

I verified your code.
What you are doing is that you are traversing the tree and when you found that all the child nodes of a three node are null you are setting it to be delete or you are deleting the parent node who’s all the child nodes are null or deleted. Till that I can understand you did right. But when there is only one child node exist and rest are null or deleted you are deleting child of it with same index, which is not understandable for me.
if(child_counter == 0) {
t->p.to_delete = TRUE;// Here you are setting the parent node to be deleted while you found that all the child nodes are null or you deleted them in the above
//but instead of this you should delete the node here itself like free(t); and you don’t need to call the method `clear_tree` again
}else if (child_counter == 1) {
t = t->child[d]; //as t is a local pointer to the current function(recursive) function. Once it return to its previous call the t in that function is holding the address of different node
free(t->child[d]); //why are you deleting this node? By doing it you are deleting the whole tree
//this child node might have a branches in it. Instead of deleting it you should store its address in parents node’s child node list or if this is the only node of the tree just leave it
t->child[d] = NULL;
}
Your problem is exactly at the above code. Just follow my comments and it might help you to fix your issue.