[leedcode 148] Sort List

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/
public class Solution {
/* Like the merge sort, we can do recursively:
(1) Split the list (slow fast pointers)
(2) sort the first part (merge sort)
(3) sort the second part (merge sort)
(4) merge the two parts (merge two sorted lists)
注意函数返回值
*/
public ListNode sortList(ListNode head) {
if(head==null||head.next==null) return head;
ListNode p= mergeSort(head);
return p;
}
public ListNode mergeSort(ListNode head){
if(head==null||head.next==null) return head;
ListNode mid=getMiddleNode(head);
ListNode p=mid.next;
mid.next=null;
ListNode first=mergeSort(head);
ListNode second=mergeSort(p);
return mergeTwoLists(first,second);
}
public ListNode getMiddleNode(ListNode head){
ListNode newHead=new ListNode(-1);
newHead.next=head;
ListNode fast=head;
ListNode slow=newHead;
while(fast!=null&&fast.next!=null){
fast=fast.next.next;
slow=slow.next;

}
return slow;
}
public ListNode mergeTwoLists(ListNode first,ListNode second){
ListNode newHead=new ListNode(-1);
ListNode p=newHead;
while(first!=null&&second!=null){
if(first.val<second.val){
p.next=first;
p=first;
first=first.next;
}else{
p.next=second;
p=second;
second=second.next;
}
}
if(first!=null){
p.next=first;
}
if(second!=null){
p.next=second;
}
return newHead.next;

}
}