1.双指针法

1.双指针法 before链表用来存放比x小的节点，after节点用来存放比x大的节点。注意要设置头节点，因为这样可以减少判断条件

  1. /*
  2.  * @lc app=leetcode.cn id=86 lang=javascript
  3.  *
  4.  * [86] 分隔链表
  5.  */
  7. // @lc code=start
  8. /**
  9.  * Definition for singly-linked list.
  10.  * function ListNode(val) {
  11.  *     this.val = val;
  12.  *     this.next = null;
  13.  * }
  14.  */
  15. /**
  16.  * @param {ListNode} head
  17.  * @param {number} x
  18.  * @return {ListNode}
  19.  */
  20. var partition = function (head, x) {
  21.   let beforeHead = (before = new ListNode())
  22.   let afterHead = (after = new ListNode())
  24.   while (head) {
  25.     if (head.val < x) {
  26.       before.next = head
  27.       before = before.next
  28.     } else {
  29.       after.next = head
  30.       after = after.next
  31.     }
  33.     head = head.next
  34.   }
  35.   after.next = null
  36.   before.next = afterHead.next
  37.   return beforeHead.next
  38. }
  39. // @lc code=end