leetcode148.排序链表

思路源自

【力扣hot100】【LeetCode 148】排序链表｜归并排序的应用

仿照归并排序 

先使用一个快慢指针将链表一分为二，然后进行排序归并

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode sortList(ListNode head) {
        if (head == null || head.next == null) {
            return head;
        } else {
            ListNode slow = head, fast = head.next;
            while (fast != null && fast.next != null) {
                slow = slow.next;
                fast = fast.next.next;
            }
            ListNode secondPart = slow.next;
            slow.next = null;
            ListNode firstPart = head;
            ListNode sortFirPart = sortList(firstPart);
            ListNode sortSecPart = sortList(secondPart);
            return sortList(sortFirPart, sortSecPart);
        }
    }

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