203. Remove Linked List Elements

不用dummy node会很麻烦，要分情况讨论，问题是为什么不用。。用的话很简单。

public class Solution {
    public ListNode removeElements(ListNode head, int val) {
        if (head == null) return head;
        ListNode dummy = new ListNode(0);
        dummy.next = head;
        ListNode temp = dummy;
        while (temp.next != null) {
            if (temp.next.val == val) {
                temp.next = temp.next.next;
            } else {
                temp = temp.next;
            }
        }
        
        return dummy.next;
    }
}