Finding lonely nodes in a binary tree

Easy solution:

This is a standard approach of implementation using pre-order traversal:

void pre_order(struct TreeNode *nd, int *res, int *rs);
int* getLonelyNodes(struct TreeNode* root, int* returnSize){
    int *res;
    
    if (!root)
        return NULL;
    
    (*returnSize) = 0;
    res = (int *)malloc(1000*sizeof(int));
    // todo check
    
    pre_order(root, res, returnSize);
    return res;
}
void pre_order(struct TreeNode *nd, int *res, int *rs){
    if (!nd)
        return;
    
    if (nd->left && !nd->right) 
        res[(*rs)++] = nd->left->val;
    if (nd->right && !nd->left)
        res[(*rs)++] = nd->right->val;
    pre_order(nd->left, res, rs);
    pre_order(nd->right, res, rs);
    return;
}