一些小问题

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode partition(ListNode head, int x) {
        ListNode leftDummy = new ListNode(0);
        ListNode rightDummy = new ListNode(0);
        ListNode left = leftDummy;
        ListNode right = rightDummy;
        while (head != null) {
            if (head.val < x) {
                left.next = head;
                left = head;
            } else {
                right.next = head;
                right = head;
            }
            head = head.next;
        }
        right.next = null;
        left.next = rightDummy.next;
        return leftDummy.next;
    }
}

这段代码不加right.next = null; 这句话就会报错

Memory Limit Exceeded 

若不置空，right.next可能指向某个链表中已经存在的结点。