Why can't I create Tree using the code below?

Question

It's a function to bulid a binary tree,but it can't work,can someone tell me what's wrong with my code

typedef struct treeNode* BT;
struct treeNode {
    int val;
    treeNode* left;
    treeNode* right;
};
BT createTree(BT root) {
    int data = 0;
    cin >> data;
    if (data == -1) {
        return nullptr;
    }
    root = new treeNode;
    root->val = data;
    createTree(root->left);
    createTree(root->right);
    return root;
}

Answer 1

You're ignoring the result of createTree(root->left); and createTree(root->right); What's more, in your implementation the argument root is meaningless. You're not using it at all. Code below should work.

BT createTree() {
    int data = 0;
    BT root;
    cin >> data;
    if (data == -1) {
        return nullptr;
    }
    root = new treeNode;
    root->val = data;
    root->left = createTree();
    root->right = createTree();
    return root;
}