C malloc to pointer to NULL not working - c

Question

I am writing a function that inserts a new node in a binary search tree. In order to avoid having too many if-else's, I am using a pointer called nodeSide that points to either node's left or right, as follows:
void insertHelper(Node *node, int val) {
Node *nodeSide;
if (val < node->val) {
nodeSide = node->left;
} else {
nodeSide = node->right;
}
if (nodeSide == NULL) {
nodeSide = (Node *)malloc(sizeof(Node));
nodeSide->val = val;
nodeSide->left = NULL;
nodeSide->right = NULL;
return;
}
else {
insertHelper(nodeSide, val);
}
}
The node however, isn't actually being added. It seems like doing this:
Node *node = malloc(...);
node->left = NULL;
Node *anotherNode = nodeLeft;
anotherNode = malloc(...);
doesn't in fact add a new node to tree. Any ideas why? The pointer should be pointing to the right place, regardless whether it is null or not. Or am I wrong here?
Here is my full code:
#include <stdlib.h>
#include <stdio.h>
typedef struct _node {
int val;
struct _node * left;
struct _node * right;
int ht;
} Node;
void insertHelper(Node *node, int val) {
Node *nodeSide;
if (val < node->val) {
nodeSide = node->left;
} else {
nodeSide = node->right;
}
if (nodeSide == NULL) {
nodeSide = (Node *)malloc(sizeof(Node));
nodeSide->val = val;
nodeSide->left = NULL;
nodeSide->right = NULL;
return;
}
else {
insertHelper(nodeSide, val);
}
}
Node * getNode(int value) {
Node * node = (Node * )malloc(sizeof(Node));
node->val = value;
node->left = NULL;
node->right = NULL;
node->ht = 0;
return node;
}
Node * getTree() {
Node *root = getNode(3);
Node *rootLeft = getNode(2);
root->left = rootLeft;
Node *rootRight = getNode(4);
root->right = rootRight;
Node *rootRightRight = getNode(5);
rootRight->right = rootRightRight;
return root;
}
int main() {
Node * root = getTree();
insertHelper(root, 6);
// to verify:
printf("%d", root->right->right->right->val);
return 0;
}

Answer 1

Your code does not store the new node that you create anywhere in your tree.
Perhaps you could do e.g.
if (nodeSide == NULL) {
nodeSide = (Node *)malloc(sizeof(Node));
nodeSide->val = val;
nodeSide->left = NULL;
nodeSide->right = NULL;
if (val < node->val)
node->left = nodeSide;
} else {
node->right = nodeSide;
}
return;
}
But you could use a double pointer, to avoid the extra if/else there:
Node **nodeSide;
if (val < node->val) {
nodeSide = &node->left;
} else {
nodeSide = &node->right;
}
if (*nodeSide == NULL) {
*nodeSide = (Node *)malloc(sizeof(Node));
(*nodeSide)->val = val;
(*nodeSide)->left = NULL;
(*nodeSide)->right = NULL;
return;
}
else {
insertHelper(*nodeSide, val);
}
}

Answer 2

Youre insertHelper seems to lack one relevant line of code just before the return:
node->left = nodeSide;
But keep in mind which side to add the value. Maybe you should track the relevant side as variable inside the function.