Data Structure Linked List: Merge Sort for Linked Lists

http://www.geeksforgeeks.org/merge-sort-for-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 *next;
     node() : data(), next(NULL) { }
     node(int d) : data(d), next(NULL) { }
 };

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

 void print(node* head) {
     if (!head) return;
     cout << head->data << " ";
     print(head->next);
 }

 void frontbacksplit(node *head, node *&a, node *&b) {
     node *p, *q;
     if (!head || !head->next) {
         a = head;
         b = NULL;
         return;
     }
     p = head;
     q = head->next;
     while (q) {
         q = q->next;
         if (q) {
             q = q->next;
             p = p->next;
         }
     }
     a = head;
     b = p->next;
     p->next = NULL;
 }

 node *sortmerge(node *a, node *b) {
     node *ans = NULL;
     if (!a) return b;
     if (!b) return a;
     if (a->data < b->data) {
         ans = a;
         ans->next = sortmerge(a->next, b);
     }
     else {
         ans = b;
         ans->next = sortmerge(a, b->next);
     }
     return ans;
 }

 void mergesort(node *&head) {
     if (!head || !head->next) return;
     node *a, *b;
     node *h = head;
     frontbacksplit(h, a, b);
     mergesort(a);
     mergesort(b);
     head = sortmerge(a, b);
 }

 int main() {
     node *head = NULL;
     push(head, );
     push(head, );
     push(head, );
     push(head, );
     push(head, );
     push(head, );
     mergesort(head);
     print(head);
     return ;
 }