lc-203

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 。

示例 1：

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：[]

提示：

• 列表中的节点数目在范围 [0, 104] 内
• 1 <= Node.val <= 50
• 0 <= val <= 50

//leetcode submit region begin(Prohibit modification and deletion)
/**
 * 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 head;
        }

        ListNode newHead = null;
        if (head.val != val) {
            newHead = head;
        }

        ListNode node = head;
        ListNode last = head;
        while (node != null) {
            if (node.val == val) {
                if (newHead != null) {
                    last.next = node.next;
                    node = node.next;
                    continue;
                }
            } else {
                if (newHead == null) {
                    newHead = node;
                }
            }

            last = node;
            node = node.next;
        }
        return newHead;
    }
}
//leetcode submit region end(Prohibit modification and deletion)