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class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        if (NULL == head)
        {
            return NULL;
        }
        ListNode * before = NULL;
        ListNode * now = head;
        while (now != NULL)
        {
            if (now -> val == val)
            {
                if (before == NULL)
                {
                    head = head->next;
                    now = head;
                }
                else
                {
                    if (now != NULL)
                    {
                        before->next = now->next;
                    }
                    now = before->next;
                }
            }
            else
            {
                before = now;
                now = now->next;
            }
        }
        return head;
    }
};