Merge Sort for Linked Lists

PROBLEM :

Sort a linked list in O(n log n) time using constant space complexity.

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SIMPLE c++ IMPLEMENTATION :
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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */

class Solution {
public:
    ListNode* sortList(ListNode* head) {
        if(head==NULL||head->next==NULL)
            return head ;
           
        ListNode* a=NULL ;
        ListNode* b=NULL ;
        divide2half(head,&a,&b) ;
        a=sortList(a) ;
        b=sortList(b) ;
        head=MeargeSort(a,b) ;
        return head ;
    }
   
    void divide2half(ListNode* head,ListNode* *a,ListNode* *b){
        ListNode* slow,*fast ;
        slow=head ;
        fast=head->next ;
       
        while(fast!=NULL&&fast->next!=NULL){
            fast=fast->next->next ;
            slow=slow->next ;
        }
       
        (*a)=head ;
        (*b)=slow->next ;
        slow->next=NULL ;
    }
   
    ListNode* MeargeSort(ListNode* a,ListNode* b){
        ListNode* final=NULL ;
       
        if(a==NULL)
            return b ;
        if(b==NULL)
            return a ;
           
        if(a->val<b->val){
            final=a ;
            final->next=MeargeSort(a->next,b) ;
        }
        else{
            final=b ;
            final->next=MeargeSort(a,b->next) ;
        }
        return final ;
    }
};

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