How to find sum of nodes in binary tree ,Can you change in the code

#1
#2 
#include<bits/stdc++.h>
using namespace std;

class node {
    public:
    int data;
    node*left;
    node*right;
    node(int d) {
        data = d;
        left = NULL;
        right = NULL;
    }
};

node* buildtree() {
  int d;
  cin>>d;
  if(d==-1){
      return NULL;
  }
   node*root = new node(d);
   root->left = buildtree();
   root->right = buildtree();
   return root;
}

void preorder(node*root) {
  if(root == NULL){
      return;
  }
  cout<<root->data<<", ";
  preorder(root->left);
  preorder(root->right);
}

void inorder(node*root) {
  if(root == NULL){
      return;
  }
  inorder(root->left);
  cout<<root->data<<", ";
  inorder(root->right);
}

void postorder(node*root) {
  if(root == NULL){
      return;
  }
  postorder(root->left);
  postorder(root->right);
  cout<<root->data<<", ";
}

//Calculating height of the tree
int height(node*root) {
    if(root == NULL){
        return 0;
    }
    int ls = height(root->left);
    int rs = height(root->right);
    return max(ls,rs) + 1;
}

//To print a single level in a binary tree
void printkthlevel(node*root, int k ) {
 if(root == NULL){
     return;
 }
  if(k==1) {
      cout<<root->data<<" ";
     return;
  }
  printkthlevel(root->left,k-1);
  printkthlevel(root->right,k-1);
  return;

}
//To print all levels in a btree O(n2)
void printalllevels(node*root) {
    if(root==NULL){
        return;
    }
    int H = height(root);
    for(int i=1;i<=H;i++){
     printkthlevel(root,i);
     cout<<endl;
    }
    return;
}

void bfs(node*root) {
    queue<node*> que;
    que.push(root);
    que.push(NULL);
    while(!que.empty()){
        node*f = que.front();
        if(f==NULL){
            cout<<endl;
            que.pop();
            if(!que.empty()){
                que.push(NULL);
            }
        }
        else{
        cout<<f->data<<" ";
        que.pop();
        if(f->left){
           que.push(f->left);
        }
        if(f->right){
         que.push(f->right);
        
        }
        }
    }
    return;
}

int count(node*root) {
    if(root == NULL){
        return 0;
    }
    return 1 + count(root->left) + count(root->right);
}
int sum(node*root){
    if(root==NULL)return 0;
    return root->data+sum(root->right)+sum(root->left);
}
int main(){
 node*root = buildtree();
/*
 preorder(root);
 cout<<endl;
 inorder(root);
 cout<<endl;
 postorder(root);
 cout<<endl;
 cout<<height(root);
 cout<<endl;
 printkthlevel(root,3);
 cout<<endl;
 printalllevels(root);
 */
 bfs(root);
 cout<<endl;
 cout<<count(root)<<endl;
 cout<<sum(root);
}
#3

Can I have explanation for that

#4

Simple recurrence relation
Sum of current node is = current_node_value + sum_of_its_children.
I will add current and rest is upto recursion to add.