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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode sortList(ListNode head) {
        if (head == null || head.next == null) return head;
        ListNode middle = findMidpoint(head);
        
        ListNode right = sortList(middle.next);
        middle.next = null;
        ListNode left = sortList(head);
        
        return merge(left,right);
        
    }
    public ListNode findMidpoint(ListNode head){
        ListNode slow = head;
        ListNode fast = head.next;
        while(fast != null && fast.next != null){
            fast = fast.next.next;
            slow = slow.next;
        }
        return slow;
    }
    public ListNode merge(ListNode left, ListNode right){
        ListNode dummy = new ListNode(-1);
        ListNode tail = dummy;
        while(left != null && right != null){
            if(left.val < right.val){
                tail.next = left;
                left = left.next;
            } else {
                tail.next = right;
                right = right.next;
            }
            tail = tail.next;
        }
        if (left != null){
            tail.next = left;
        } else {
            tail.next = right;
        }
        return dummy.next;
    }
}