Given the head of a linked list and an integer val, remove all the nodes of the linked list that has Node.val == val, and return the new head.
Example1:

Input: head = [1,2,6,3,4,5,6], val = 6
Output: [1,2,3,4,5]

Example2:

Input: head = [], val = 1
Output: []

Example3:

Input: head = [7,7,7,7], val = 7
Output: []

这道题没有什么算法，就是在写list的时候注意边界条件。多多练习，就算题目简单，刚开始也会有很多错误。

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode removeElements(ListNode head, int val) {
        if (head == null) {
            return null;
        }
        ListNode dummy = new ListNode(0, head);
        ListNode next = head;
        head = dummy;
        while (next != null) {
            while (next != null && next.val == val) {
                next = next.next;
            }
            head.next = next;            
            if (next != null) {
                next = next.next;  
                head = head.next;
            }
        }
        return dummy.next;
    }
}