# TLE in sortlist merge sort c++

• Recursive merge sort function and merge function to join sub lists causing TLE

``````  class Solution {
public:
ListNode *sortList(ListNode *head) {
if(head==NULL||head->next==NULL)
return head;
ListNode * mid=split(head);
head=sortList(head);
mid=sortList(mid);
ListNode* ret=  merge(head,mid);
return ret;

}
ListNode* split(ListNode * root){
ListNode * ret=root->next;
ListNode * temp=ret;
while(root->next=NULL&&root->next->next!=NULL){
root->next=root->next->next;
temp->next=root->next->next;
root=root->next;
temp=temp->next;
}
root->next=NULL;

return ret;
}
ListNode * merge(ListNode * head,ListNode * begin){
ListNode  temp(-1);
ListNode * target=&temp;
while(head!=NULL&&begin!=NULL){
if(head->val<=begin->val){
target->next=head;
head=head->next;
target=target->next;
}
else{
target->next=begin;
begin=begin->next;
target=target->next;
}
}
if(head!=NULL)
target->next=head;
else
if(begin!=NULL)
target->next=begin;
return temp.next;

}

};``````

• Run time error Time Limit Exceeded is due to an infinite loop caused by a wrong boolean condition inside the first while loop, it must be while(root->next!=NULL&&root->next->next!=NULL) but inspite of it while(root->next=NULL&&root->next->next!=NULL) is typed. Update the code with this change at line 16.

``````  class Solution {
public:
ListNode *sortList(ListNode *head) {
if(head==NULL||head->next==NULL)
return head;
ListNode * mid=split(head);
head=sortList(head);
mid=sortList(mid);
ListNode* ret=  merge(head,mid);
return ret;

}
ListNode* split(ListNode * root){
ListNode * ret=root->next;
ListNode * temp=ret;
while(root->next!=NULL&&root->next->next!=NULL){
root->next=root->next->next;
temp->next=root->next->next;
root=root->next;
temp=temp->next;
}
root->next=NULL;

return ret;
}
ListNode * merge(ListNode * head,ListNode * begin){
ListNode  temp(-1);
ListNode * target=&temp;
while(head!=NULL&&begin!=NULL){
if(head->val<=begin->val){
target->next=head;
head=head->next;
target=target->next;
}
else{
target->next=begin;
begin=begin->next;
target=target->next;
}
}
if(head!=NULL)
target->next=head;
else
if(begin!=NULL)
target->next=begin;
return temp.next;

}

};``````

• I have a simple solution, may be simplest. see below:

``````ListNode * sortList(ListNode *head) {
if (!head || !head->next) return head;

ListNode *mid = head, *temp = head->next;
while (temp && temp->next) {
mid = mid->next;
temp = temp->next->next;
}

temp = mid->next;
mid->next = NULL;
ListNode *list2 = sortList(temp);
ListNode *list1 = sortList(head);

ListNode **curr = &list1;
while (list2 && *curr) {
ListNode *entry = *curr;
if (list2->val < entry->val) {
*curr = list2;
list2 = entry;
}
curr = &(*curr)->next;
}
if (list2) *curr = list2;

return list1;
}
``````

Firstly, find the middle element and split the link list. Then merge two sorted list, which called list1 and list2.Instead of merge list1 and list2 to a new list, I choose insert every element in list2 to list1. curr is the pointer to last merged element's NEXT pointer.

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