Linked list merge sort

mergeSortLL.cpp

#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include<bits/stdc++.h>
using namespace std;

struct ListNode {
    int val;
    ListNode* next = nullptr;
    ListNode(int data):val(data){}
};

ListNode*nn(int d){return new ListNode(d);}

void display(ListNode* head){
    while(head){
        cout<<head->val<<" ";
        head=head->next;
    }
    cout<<'\n';
}

ListNode* merge(ListNode* left, ListNode* right) {
    ListNode result(0);
    ListNode* cur = &result;
    while (left || right) {
        if (!right) {
            cur->next = nn(left->val);
            left = left->next;
        }
        else if (!left) {
            cur->next = nn(right->val);
            right = right->next;
        }
        else if (left->val <= right->val) {;
            cur->next = nn(left->val);
            left = left->next;
        } else {
            cur->next = nn(right->val);
            right = right->next;
        }
        cur = cur->next;
    }
    return result.next;
}

/*
ListNode* merge(ListNode* left, ListNode* right) {
    ListNode result(0);
    ListNode* cur = &result;
    while (left || right) {
        if (!right || (left && left->val <= right->val)) {
            cur->next = nn(left->val);
            left = left->next;
        }
        else if (!left || (right && right->val <= left->val)) {
            cur->next = nn(right->val);
            right = right->next;
        }
        cur = cur->next;
    }
    return result.next;
}
*/

ListNode* mergeSort(ListNode* head) {
    if (!head || !head->next)
        return head;
    
    ListNode* slow = head, *fast = head->next;
    while (fast && fast->next) {
        slow = slow->next;
        fast = fast->next->next;
    }
    
    ListNode* right = slow->next;
    slow->next = nullptr;
    
    return merge(mergeSort(head), mergeSort(right));
}


int main() {
    ListNode*head=nn(5);
    head->next=nn(4);
    head->next->next = nn(3);
    head->next->next->next = nn(2);
    head->next->next->next->next = nn(1);
    display(head);
    head = mergeSort(head);
    display(head);
}