LeetCode--insertion-sort-list

class Solution {
public:
ListNode* findMiddle(ListNode* head){
ListNode* chaser = head;
ListNode* runner = head->next;
while(runner != NULL && runner->next != NULL){
chaser = chaser->next;
runner = runner->next->next;
}
return chaser;
}

ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
if(l1 == NULL){
return l2;
}
if(l2 == NULL){
return l1;
}
ListNode* dummy = new ListNode(0);
ListNode* head = dummy;
while(l1 != NULL && l2 != NULL){
if(l1->val > l2->val){
head->next = l2;
l2 = l2->next;
}
else{
head->next = l1;
l1 = l1->next;
}
head = head->next;
}
if(l1 == NULL){
head ->next = l2;
}
if(l2 == NULL){
head->next = l1;
}
return dummy->next;
}

ListNode* sortList(ListNode* head) {
if(head == NULL || head ->next == NULL){
return head;
}
ListNode* middle = findMiddle(head);
ListNode* right = sortList(middle->next);
middle -> next = NULL;
ListNode* left = sortList(head);
return mergeTwoLists(left, right);
}
};