給定一個鏈表和一個特定值 x，對鏈表進行分隔，使得所有小于 x 的節點都在大于或等于 x 的節點之前。

你應當保留兩個分區中每個節點的初始相對位置。

示例:

輸入: head = 1->4->3->2->5->2, x = 3
輸出: 1->2->2->4->3->5

思路：分別創建兩個鏈表保存兩種節點，最后連起來即可。

/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode(int x) { val = x; }* }*/
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;}
}

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