Sort List

Merge Sort.

public class Solution {
    public ListNode sortList(ListNode head) {
        if(head == null || head.next == null) return head;
        ListNode slow = head, fast = head;
        while(fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        fast = slow.next;
        slow.next = null;
        slow = head;
        ListNode first = sortList(slow);
        ListNode second = sortList(fast);
        return merge(first, second);
    }
    
    private ListNode merge(ListNode first, ListNode second) {
        ListNode dummy = new ListNode(0);
        ListNode head = dummy;
        while(first != null && second != null) {
            if(first.val < second.val) {
                ListNode next = first.next;
                head.next = first;
                head = head.next;
                head.next = null;
                first = next;
            } else {
                ListNode next = second.next;
                head.next = second;
                head = head.next;
                head.next = null;
                second = next;
            }
        }
        if(first != null) {
            head.next = first;
        } else {
            head.next = second;
        }
        return dummy.next;
    }
}