Linked-list MergeSort

MergeSort for Linkedlist

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* sortList(ListNode* head) {
    	if(head==NULL || head->next==NULL){
      		return head;  	
        }
        ListNode* slow = head, *fast = head;
        ListNode* temp = NULL;
        
        while(fast!=NULL && fast->next!=NULL){
        	temp = slow;
            slow = slow->next;
            fast = fast->next->next;
        }
        temp->next = NULL;
        ListNode* l1 = sortList(head);
        ListNode* l2 = sortList(slow);
        
        return mergeList(l1, l2);
    }
    
    ListNode* mergeList(ListNode* l1, ListNode* l2) {
    	ListNode* base = new ListNode(0);
        ListNode* cur = base;
    	while(l1!=NULL && l2!=NULL){
        	if(l1->val<=l2->val){
            	cur->next = l1;
                l1 = l1->next;
            }else{
            	cur->next = l2;
                l2 = l2->next;
            }
            cur = cur->next;
    	}
        
        if(l1!=NULL){
        	cur->next = l1;
        }
        if(l2!=NULL){
        	cur->next = l2;
        }
		return base->next;
	}
};

• https://leetcode.com/problems/sort-list/
• time-complexity : o(nlogn)
• space-complexity : o(1)