binary tree preorder traversal

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
/**
 * Return an array of size *returnSize.
 * Note: The returned array must be malloced, assume caller calls free().
 */
int* preorderTraversal(struct TreeNode* root, int* returnSize) {
    int i = 0;
    //int *arr = (int *)malloc(100*sizeof(int));
    if(root)
    {
        returnSize[iPP] = root->val;
        preorderTraversal(root->left, returnSize);
        preorderTraversal(root->right, returnSize);
    }
    
    return returnSize;
}