class Solution {

public:

    ListNode* removeElements(ListNode* head, int val) {

        if(head!=NULL && head->val==val)

        {

            head=head->next;

        }

        if(head==NULL) return NULL;

        //处理第一位是val的情况

        while(head->val==val)

        {

            head=head->next;

            if(head==NULL) return NULL;

        }

        ListNode *curr=head->next;

        ListNode *pre=head;

        while(curr!=NULL)

        {

            if(curr->val==val)

            {

                pre->next=curr->next;

            }

            else

                pre=curr;

            curr=curr->next;

        }

        return head;

    }

};

class Solution {

public:

    ListNode* removeElements(ListNode* head, int val) {

        ListNode* vir=new ListNode(-);

        vir->next=head;

        ListNode *pre=vir;

        while(pre->next!=NULL)

        {

            if(pre->next->val==val)

            {

                pre->next=pre->next->next;

            }

            else

                pre=pre->next;

        }

        return vir->next;

    }

};

class Solution {

    public ListNode removeElements(ListNode head, int val) {

       if(head==null)

           return null;

        head.next=removeElements(head.next,val);

        if(head.val==val){

            return head.next;

        }else{

            return head;

        }

    }

}