1.直接使用原来的链表来进行删除操作。

class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        // 删除头结点
        while (head != NULL && head->val == val) {
            ListNode* tmp = head;
            head = head->next;
            delete tmp;
        }
        // 删除非头结点
        ListNode* cur = head;
        while (cur != NULL && cur->next != NULL) {
            if (cur->next->val == val) {
                ListNode* tmp = cur->next;
                cur->next = cur->next->next;
                delete tmp;
            } else {
                cur = cur->next;
            }
        }

        return head;
    }
};

2.设置一个虚拟头结点在进行删除操作。

class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        ListNode* dummy = new ListNode(0);
        dummy->next = head;
        ListNode* cur = dummy;

        while(cur->next != NULL){
            if(cur->next->val == val){
                ListNode* tmp = cur->next;
                cur->next = cur->next->next;
                delete tmp;
            }else{
                cur = cur->next;
            }
        }

        return dummy->next;
    }
};

3.采用前后指针的方式

class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        ListNode* dummy = new ListNode(0);
        dummy->next = head;
        ListNode *cur = head, *pre = dummy;

        while (cur) {
            if (cur->val == val) {
                // ListNode* tmp = cur;
                pre->next = cur->next;
                cur = cur->next;
                // delete tmp;
            } else {
                pre = pre->next;
                cur = cur->next;
            }
        }

        return dummy->next;
    }
};