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 = 0) : 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, 30 );
    push( root, 8 );
    push( root, 7 );
    push( root, 5 );

    push( ( root->right ), 20 );
    push( ( root->right ), 10 );

    push( ( root->right->right ), 50 );
    push( ( root->right->right ), 22 );
    push( ( root->right->right ), 19 );

    push( ( root->right->right->right ), 45 );
    push( ( root->right->right->right ), 40 );
    push( ( root->right->right->right ), 35 );
    push( ( root->right->right->right ), 20 );

    root = flatten(root);
    print(root);
    return 0;
}