SortList.java

// https://leetcode.com/problems/sort-list
// T: O(N log(N))
// S: O(log(N))

public class SortList {
    public ListNode sortList(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode mid = getMid(head);
        ListNode left = sortList(head);
        ListNode right = sortList(mid);
        return merge(left, right);
    }

    ListNode merge(ListNode list1, ListNode list2) {
        ListNode dummyHead = new ListNode();
        ListNode tail = dummyHead;
        while (list1 != null && list2 != null) {
            if (list1.val < list2.val) {
                tail.next = list1;
                list1 = list1.next;
            } else {
                tail.next = list2;
                list2 = list2.next;
            }
            tail = tail.next;
        }
        tail.next = (list1 != null ? list1 : list2);
        return dummyHead.next;
    }

    ListNode getMid(ListNode head) {
        ListNode midPrev = null;
        while (head != null && head.next != null) {
            midPrev = (midPrev == null ? head : midPrev.next);
            head = head.next.next;
        }
        ListNode mid = midPrev.next;
        midPrev.next = null;
        return mid;
    }
}