分隔链表

// Java Stream Solution
// 注意：Java的Stream不适用于链表操作，因此此方法使用常规Java代码。
// 思路：与普通Java解法相同，无法使用Stream进行链表操作。
class ListNode {
    int val;
    ListNode next;
    ListNode(int val) { this.val = val; }
}
 
class Solution {
    public ListNode partition(ListNode head, int x) {
        ListNode smallerHead = new ListNode(0);
        ListNode greaterHead = new ListNode(0);
        ListNode smaller = smallerHead, greater = greaterHead;
        while (head != null) {
            if (head.val < x) {
                smaller.next = head;
                smaller = smaller.next;
            } else {
                greater.next = head;
                greater = greater.next;
            }
            head = head.next;
        }
        greater.next = null;
        smaller.next = greaterHead.next;
        return smallerHead.next;
    }
}