C：释放二进制搜索树

Question

I have this code:

node* free_tree(node *root){

  if(root != NULL){

    free_tree(root->left);
    free_tree(root->right);

    free(root->name);
    free(root);
  }
  return NULL;
}

I know this isn't correct, correct version is with:

root -> left = free_tree(root->left);
root -> right = free_tree(root->right);

What I don't understand is, why does this work? When I return from free_tree(root->left) with NULL, my function require that some node* receive NULL value, this is not the case here, so I don't get it, why does this work? Why is this not a compilation error?

Answer 1

A couple things:

1. There's no reason why this function needs to return anything. The second version you show uses the return value to update the left and right pointers prior to freeing them, but (a) There's no need to since you're freeing the node anyway, and (b) The value is always NULL . Since it's a constant, there's no reason to return it.

2. There's no compilation error because there are no type violations. The only issue is that you're calling a function that returns a node * but aren't using the return value. However, that's legal C.

Answer 2

You are deleting the entire tree, and apart from the root pointer which should be set to NULL (or not even that, depending on the implementation), the members of nodes don't have to be set NULL.

Your correct code is identical to:

node* free_tree(node *root){

  if(root != NULL){

    free_tree(root->left);
    free_tree(root->right);

    root->left = NULL;
    root->right = NULL;

    free(root->name);
    free(root);
  }
  return NULL;
}

As you can see right after the members left and rigth are set to NULL, the entire node is free d. So their values don't matter as far as the correctness of the program is concerned.

Answer 3

The functions doesn't require assignment. The value is just thrown away if it is not needed.

int OutputSquareValue(int value) {
   int result = value * value;
   printf("%d", result);
   return result;
}

If you don't need the result in your code, but only want it to be outputed you go

OutputSquareValue(5);

And it is perfectly ok. If you need to use your new value, you go

printf("Squre of %d = ", x);
y = OutputSquareValue(x);
printf("%d^4 =", x);
OutputSquareValue(y);