牛客网：NC70 单链表的排序

import java.util.*;

/*
 * public class ListNode {
 *   int val;
 *   ListNode next = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param head ListNode类 the head node
     * @return ListNode类
     */
    public ListNode sortInList (ListNode head) {
        // write code here
        if (head == null || head.next == null){
            return head;
        }
        
        // 左指针
        ListNode left = head;
        // 中间指针,需要先走一步
        ListNode mid = head.next;
        ListNode right = head.next.next;
        while (right != null && right.next != null){
            left = left.next;
            mid = mid.next;
            right = right.next.next;
        }
        left.next = null;
        return buildListNode(sortInList(head), sortInList(mid));
    }
    
    private ListNode buildListNode(ListNode node1, ListNode node2){
        if (node1 == null ){
            return node2;
        }
        if (node2 == null){
            return node1;
        }
        ListNode dummy = new ListNode(0);
        ListNode cur = dummy;
        while (node1 != null && node2 != null){
            if (node1.val <= node2.val){
                cur.next = node1;
                node1 = node1.next;
            } else{
                cur.next = node2;
                node2 = node2.next;
            }
            cur = cur.next;
        }
        cur.next = node1 == null ? node2 : node1;
        return dummy.next;
    }
}