Data Structure Linked List: Flattening a Linked List

http://www.geeksforgeeks.org/flattening-a-linked-list/

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

 struct node {
     int data;
     node *right;
     node *down;
     node(int d = ) : data(d), right(NULL), down(NULL) { }
 };

 struct cmp {
     bool operator() (node *a, node *b) {
         return a->data > b->data;
     }
 };

 void push(node* &head, int k) {
     node *new_node = new node(k);
     new_node->down = head;
     head = new_node;
 }

 node* flatten(node *&root) {
     priority_queue<node*, vector<node*>, cmp> S;
     while (root) {
         S.push(root);
         root = root->right;
     }
     root = NULL;
     node *p;
     while (!S.empty()) {
         node *top = S.top();
         S.pop();
         if (!root) {
             root = top;
             p = top;
         }
         else {
             p->right = top;
             p = p->right;
         }
         if (top->down) S.push(top->down);
     }
     return root;
 }

 void print(node *head) {
     while (head) {
         cout << head->data << " ";
         head = head->right;
     }
 }

 int main() {
     node *root = NULL;
     push( root,  );
     push( root,  );
     push( root,  );
     push( root,  );

     push( ( root->right ),  );
     push( ( root->right ),  );

     push( ( root->right->right ),  );
     push( ( root->right->right ),  );
     push( ( root->right->right ),  );

     push( ( root->right->right->right ),  );
     push( ( root->right->right->right ),  );
     push( ( root->right->right->right ),  );
     push( ( root->right->right->right ),  );

     root = flatten(root);
     print(root);
     return ;
 }