Solution.java

package $148;

import datastruc.ListNode;

/**
 * @author Junlan Shuai[shuaijunlan@gmail.com].
 * @date Created on 10:42 AM 2018/08/13.
 */
public class Solution {
    public ListNode sortList(ListNode head){
        if (head == null || head.next == null){
            return head;
        }
        ListNode prev = null, slow = head, fast = head;
        while (fast != null && fast.next != null){
            prev = slow;
            slow = slow.next;
            fast = fast.next.next;
        }
        //cut into two lists
        prev.next = null;

        ListNode l1 = sortList(head);
        ListNode l2 = sortList(slow);

        return merge(l1, l2);
    }

    private ListNode merge(ListNode l1, ListNode l2){
        ListNode node = new ListNode(0);
        ListNode temp = node;
        while (l1 != null && l2 != null){
            if (l1.val < l2.val){
                temp.next = l1;
                l1 = l1.next;
            }else {
                temp.next = l2;
                l2 = l2.next;
            }
            temp = temp.next;
        }
        if (l1 != null){
            temp.next = l1;
        }
        if (l2 != null){
            temp.next = l2;
        }
        return node.next;
    }
}