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(0), 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, 15);
    push(head, 10);
    push(head, 5);
    push(head, 20);
    push(head, 3);
    push(head, 2);
    mergesort(head);
    print(head);
    return 0;
}