Data Structure Binary Tree: Connect nodes at same level using constant extra space

http://www.geeksforgeeks.org/connect-nodes-at-same-level-with-o1-extra-space/

recursive:

 #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) { }
 };

 node *getnext(node *root) {
     node *next = root->next;
     while (next) {
         if (next->left) return next->left;
         if (next->right) return next->right;
         next = next->next;
     }
     return NULL;
 }

 void _connect(node *root) {
     if (!root) return;
     if (root->next) _connect(root->next);
     if (root->left) {
         if (root->right) {
             root->left->next = root->right;
             root->right->next = getnext(root);
         }
         else root->left->next = getnext(root);
         _connect(root->left);
     }
     else if (root->right) {
         root->right->next = getnext(root);
         _connect(root->right);
     }
     else _connect(root->next);
 }

 void connect(node *root) {
     if (!root) return;
     root->next = NULL;
     _connect(root);
 }

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

iterative(better)

 #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) { }
 };

 node *getnext(node *root) {
     node *next = root->next;
     while (next) {
         if (next->left) return next->left;
         if (next->right) return next->right;
         next = next->next;
     }
     return NULL;
 }

 void connect(node *root) {
     if (!root) return;
     root->next = NULL;
     while (root) {
         node *p = root;
         while (p) {
             if (p->left) {
                 if (p->right) p->left->next = p->right;
                 else p->left->next = getnext(p);
             }
             if (p->right) p->right->next = getnext(p);
             p = p->next;
         }
         if (root->left) root = root->left;
         else if (root->right) root = root->right;
         else root = root->next;
     }
 }

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