Question

I wrote a function to nullify all the leaves of a BST. The BST has a left and right pointer of course and a char called data to store the value of the node.

void removeLeaves(struct Tree* T){
  if(T->left == NULL && T->right == NULL){
    printf("removing %c\n", T->data);
    T=NULL;
  }
  else{
    if(T->left!=NULL){
      removeLeaves(T->left);
    }
    if(T->right!=NULL){  
      removeLeaves(T->right);
    }
  }  
}

I Print the tree before and after envoking this function.
And although the print statement above works and prints the nodes that are nullified, the resulting tree is the same.
I have something like:

print(BST);
removeLeaves(BST);
print(BST);

Any idea what’s going on?
Thanks.

Answer 1

T=NULL; assigns null to a local pointer, not to anything in your tree. You need to use a struct Tree ** so that you can modify the struct Tree *:

void removeLeaves(struct Tree ** T){
  if((*T)->left == NULL && (*T)->right == NULL){
    printf("removing %c\n", (*T)->data);
    *T = NULL;
  }
  else{
    if((*T)->left!=NULL){
      pruneTree((*T)->left);
    }
    if((*T)->right!=NULL){  
      pruneTree((*T)->right);
    }
  }  
}