86. Partition List 

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class Solution {
    public ListNode partition(ListNode head, int x) {
        ListNode dummyLeftHead = new ListNode();
        ListNode dummyRightHead = new ListNode();

        ListNode leftNode = dummyLeftHead;
        ListNode rightNode = dummyRightHead;
        while (head != null) {
            if (head.val < x) {
                leftNode.next = head;
                leftNode = leftNode.next;
            } else {
                rightNode.next = head;
                rightNode = rightNode.next;
            }

            head = head.next;
        }

        leftNode.next = dummyRightHead.next;
        rightNode.next = null; // 注意不要忘记切断连接

        return dummyLeftHead.next;
    }
}

References

86. Partition List