Binary tree creation and recursive traversal

#include<iostream>
#include<vector>
using namespace std;
//same as linked list data structure
struct TreeNode{
  
    int value;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x):value(x),left(NULL),right(NULL){}
};

void BST(TreeNode* node, TreeNode* insertnode){
    // Determine if the binary tree is empty
/*
    if(node == NULL){
        node = insertnode;
    }
*/
    if(node->value > insertnode->value){
    // insert left
        if(node->left == NULL){
           node->left = insertnode;
        }else{
           BST(node->left, insertnode);
        }
    }else{
    // insert right
        if(node->right == NULL){
            node->right = insertnode;
        }else{
            BST(node->right, insertnode);
        }
    }

}

//Pre-order traversal first print and node, left and right
void preorder(TreeNode* node, int layer){
    if(!node){
        return; // void type, cannot return 0;
    }
    
    for(int i = 0; i <layer; i++ ){
        cout<<"------";
    }
    cout<<node->value<<endl;

    preorder(node->left, layer++);
    preorder(node->right, layer++);
}

void postorder(TreeNode* node, int layer){
    if(!node){
        return; // void type, cannot return 0;
    }
    
    postorder(node->left, layer++);
    postorder(node->right, layer++);

    for(int i = 0; i <layer; i++ ){
        cout<<"------";
    }
    cout<<node->value<<endl;
}
void inorder(TreeNode* node, int layer){  //中序
    if(!node){
        return; // void type, cannot return 0;
    }
    
    inorder(node->left, layer++);

    for(int i = 0; i <layer; i++ ){
        cout<<"------";
    }
    cout<<node->value<<endl;

    inorder(node->right, layer++);

}
int main(){
    TreeNode node(8);
    std::vector<TreeNode*>  node_vec;
    int test[] = {3, 10, 1, 6, 15};
//Assignment of dynamic array
    for(int i = 0; i < 5; i++){
        node_vec.push_back( new TreeNode(test[i])); //

    }

    for(int i = 0; i < node_vec.size(); i++){
        BST(&node,node_vec[i]);
    }
    
    preorder(&node, 0);
    postorder(&node, 0);
    inorder(&node, 0);
return 0;
}