Data Structure Binary Tree: Populate Inorder Successor for all nodes

http://www.geeksforgeeks.org/populate-inorder-successor-for-all-nodes/

 #include <iostream>
 #include <vector>
 #include <algorithm>
 #include <queue>
 #include <stack>
 #include <string>
 #include <fstream>
 using namespace std;

 struct node {
     int data;
     struct node *left, *right, *next;
     node() : data(), left(NULL), right(NULL), next(NULL) { }
     node(int d) : data(d), left(NULL), right(NULL), next(NULL) { }
 };

 void populate(node *root) {
     static node *next = NULL;
     if (!root) return;
     populate(root->right);
     root->next = next;
     next = root;
     populate(root->left);
 }

 int main() {
     node *root = new node();
     root->left = new node();
     root->right = new node();
     root->left->left = new node();
     populate(root);
     cout << root->data << "->" << (root->next? root->next->data : -) << endl;
     cout << root->left->data << "->" << (root->left->next? root->left->next->data : -) << endl;
     cout << root->right->data << "->" << (root->right->next? root->right->next->data : -) << endl;
     cout << root->left->left->data << "->" << (root->left->left->next? root->left->left->next->data : -) << endl;
     return ;
 }