public class leetcode148 {
    public static class ListNode {
      int val;
      ListNode next;
      ListNode(int x) { val = x; }
  }
    public static void main(String[] args) {

    }

    public ListNode sortList(ListNode head) {
        if(head == null || head.next == null) return head;
        ListNode slow = head;
        ListNode fast = head.next;
        while(fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        ListNode temp = slow.next;
        slow.next = null;
        ListNode left = sortList(head);
        ListNode right = sortList(temp);
        ListNode h = new ListNode(0);
        ListNode res = h;
        while(left != null && right != null) {
            if(left.val <= right.val) {
                h.next = left;
                left = left.next;
            } else {
                h.next = right;
                right = right.next;
            }
            h = h.next;
        }
        h.next = left== null? right : left;
        return res.next;
    }
}