148. 排序链表

 归并排序

class Solution {
    public ListNode sortList(ListNode head) {
        if(head == null || head.next == null) return head;
        ListNode fast = head,slow = head;
        while(fast.next != null && fast.next.next != null) {
            fast = fast.next.next;
            slow = slow.next;
        }
        ListNode next = slow.next;
        slow.next = null;
        ListNode left = sortList(head);
        ListNode right = sortList(next);
        return merge(left,right);
    }
    public ListNode merge(ListNode l, ListNode r) {
        if(l == null) return r;
        if(r == null) return l;
        if(l.val < r.val) {
            l.next = merge(l.next,r);
            return l;
        }
        r.next = merge(l,r.next);
        return r;
    }
}