针对链表的排序：通过快慢指针找到中点，使用归并排序

    1. class Solution {
    2.     public ListNode sortList(ListNode head) {
    3.         return mergeSort(head);
    4.     }
    5.     public ListNode mergeSort(ListNode head) {
    6.         if(head == null || head.next == null ) return head;
    7.         ListNode fast = head;
    8.         ListNode dummy = new ListNode();
    9.         dummy.next = head;
    10.         ListNode slow = dummy;
    11.         while(fast != null && fast.next != null){
    12.             slow = slow.next;
    13.             fast = fast.next.next;
    14.         }
    15.         ListNode newHead = slow.next;
    16.         slow.next = null;
    17.         ListNode l1 = mergeSort(head);
    18.         ListNode l2 = mergeSort(newHead);
    19.         return merge(l1,l2);
    20.     }
    22.     ListNode merge(ListNode l1,ListNode l2){
    23.         ListNode dummy = new ListNode();
    24.         ListNode cur = null;
    25.         cur = dummy;
    26.         while(l1 != null && l2 != null){
    27.             if(l1.val > l2.val){
    28.                 cur.next = l2;
    29.                 l2 = l2.next;
    30.             }else{
    31.                 cur.next = l1;
    32.                 l1 = l1.next;
    33.             }
    34.             cur = cur.next;
    35.         }
    36.         cur.next = l1 == null ? l2 : l1;
    37.         return dummy.next;
    38.     }
    39. }