[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;
        
    }
}