Algorithms—148.Sort List

思路：归并排序。

/**
* 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 mid=getMid(head);
ListNode next=mid.next;
mid.next=null;
return merge(sortList(head),sortList(next));
}
public ListNode getMid(ListNode head){
ListNode fast=head;
ListNode slow=head;
while(fast.next!=null&&fast.next.next!=null) {
slow = slow.next;
fast = fast.next.next;
}
return slow;
}
public ListNode merge(ListNode a,ListNode b){
ListNode ans=new ListNode(0);
ListNode node=ans;
while (a!=null&&b!=null) {
if (a.val<b.val) {
node.next=a;
a=a.next;
}else {
node.next=b;
b=b.next;
}
node=node.next;
}
node.next=a==null?b:a;
return ans.next;
}
}