原题

给定一个链表和一个特定值 x，对链表进行分隔，使得所有小于 x 的节点都在大于或等于 x 的节点之前。

你应当保留两个分区中每个节点的初始相对位置。

示例:

输入: head = 1->4->3->2->5->2, x = 3
输出: 1->2->2->4->3->5

//分割链表(链表不带头结点)
//算法实现时可以使用头结点，最后返回结果时去掉

class Solution {
    public ListNode partition(ListNode head, int x) {

        ListNode before_head = new ListNode(0);
        ListNode before = before_head;
        ListNode after_head = new ListNode(0);
        ListNode after = after_head;

        while (head != null) {
            if (head.val < x) {
                before.next = head;
                before = before.next;
            } else {
                after.next = head;
                after = after.next;
            }

            head = head.next;
        }

        after.next = null;
        before.next = after_head.next;

        return before_head.next;
    }
}

参考链接：https://leetcode-cn.com/problems/partition-list/solution/fen-ge-lian-biao-by-leetcode/