There are many merge-sort solutions at the forum, but very few quicksort solutions. So I post my accepted quicksort solution here.

Well, after reading the problem statement, I intuitively select quicksort since it is able to give an in-place solution and thus costs only constant space. Also, it is `O(nlogn)`

in the expected case though it may become `O(n^2)`

in the worst case.

Then I implement my quicksort solution and test it. I then submit it to the online judge. However, the annoying TLE error occurred. I check for the forums and some people suggested to use random pivoting or duplicate skipping. However, implementing random pivoting is a little costly, I lazily tried to skip the duplicates. And it works! So now comes the following solution . Note that each time I choose the first node as the pivot. Moreover, I create a `new_head`

that points to `head`

for convenience.

Of course, this solution passes the online judge luckily. If the linked list is like: `100000 -> 99999 -> 99998 -> ... -> 1`

, it will fail since the subproblems only decrease by 1 at each recursion. However, it seems that the LeetCode OJ does not have this kind of test cases.

```
void sortListHelper(ListNode* head, ListNode* tail) {
if (head -> next == tail) return;
/* Partition the list. */
ListNode* pre = head;
ListNode* cur = head -> next;
ListNode* pivot = cur;
while (cur -> next && cur -> next != tail) {
if (pivot -> val > cur -> next -> val) {
ListNode* temp = pre -> next;
pre -> next = cur -> next;
cur -> next = cur -> next -> next;
pre -> next -> next = temp;
}
else cur = cur -> next;
}
sortListHelper(head, pivot);
/* Here is the trick. */
while (pivot -> next != tail && pivot -> next -> val == pivot -> val)
pivot = pivot -> next;
if (pivot -> next != tail) sortListHelper(pivot, tail);
}
ListNode* sortList(ListNode* head) {
ListNode* new_head = new ListNode(0);
new_head -> next = head;
sortListHelper(new_head, NULL);
return new_head -> next;
}
```