leetcode203. 移除链表元素

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(-1);
        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;
        }
    }
}