给你一个链表的头节点 head 和一个整数 val ，请你删除链表中所有满足 Node.val == val 的节点，并返回 新的头节点 。
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    输入：head = [1,2,6,3,4,5,6], val = 6
    输出：[1,2,3,4,5]

    示例 2：
    输入：head = [], val = 1
    输出：[]

    示例 3：
    输入：head = [7,7,7,7], val = 7
    输出：[]

    1. /**
    2.  * Definition for singly-linked list.
    3.  * public class ListNode {
    4.  *     int val;
    5.  *     ListNode next;
    6.  *     ListNode() {}
    7.  *     ListNode(int val) { this.val = val; }
    8.  *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
    9.  * }
    10.  */
    11. class Solution {  
    12.     public ListNode removeElements(ListNode head, int val) {
    13.         ListNode dummy = new ListNode(0);
    14.         dummy.next = head;
    15.         ListNode prev = dummy;
    16.         while(head != null){
    17.             if(head.val == val){
    18.                 prev.next = head.next;
    19.                 head = head.next;
    20.             }else{
    21.                 prev = head;
    22.                 head = head.next;
    23.             }
    24.         }
    25.     return dummy.next;
    26.     }
    27. }