题目链接/文章讲解/视频讲解：https://programmercarl.com/0707.%E8%AE%BE%E8%AE%A1%E9%93%BE%E8%A1%A8.html

CPP

class MyLinkedList {
public:
    struct ListNode {
        int val;
        ListNode* next;
        ListNode(int val):val(val), next(nullptr){}
    };

    MyLinkedList() {
        this->_dummyHead = new ListNode(0);
        this->_size = 0;
    }
    
    int get(int index) {
        if(index >= _size || index < 0) return -1; // 修正了条件
        ListNode *cur = _dummyHead;
        
        while(index--) {
            cur = cur->next;
        }
        return cur->next->val;  // cur->next 必然存在
    }
    
    void addAtHead(int val) {
        ListNode *newNode = new ListNode(val);
        newNode ->next = _dummyHead->next;
        _dummyHead->next = newNode;
        _size++;
    }
    
    void addAtTail(int val) {
        ListNode *cur = _dummyHead;
        ListNode *newNode = new ListNode(val);
        while(cur->next != nullptr){
            cur = cur->next;
        }
        cur->next = newNode;
        newNode->next = nullptr;
        _size++;
    }
    
    void addAtIndex(int index, int val) {
        if(index >_size) return;
        if(index<0) index =0;
        ListNode *cur = _dummyHead;
        ListNode *newNode = new ListNode(val);
        if(index == _size){
            while(cur->next != nullptr){
                cur = cur->next;
            }
            cur->next = newNode;
            newNode ->next= nullptr;
        }else{
            while(index--){
                cur = cur->next;
            }
            ListNode *temp = new ListNode(0);
            temp = cur->next;
            cur->next =newNode;
            newNode->next = temp;
        }
        _size++;
    }
    
    void deleteAtIndex(int index) {
        if(index >_size ||index<0)return;
        ListNode * cur = _dummyHead;
        while(index--){
            cur = cur->next;
        }
        ListNode *temp = cur->next;
        cur->next = cur->next->next;
        delete temp;
        temp = nullptr;
        _size--;
    }
    private:
    int _size;
    ListNode *_dummyHead;
};

/**
 * Your MyLinkedList object will be instantiated and called as such:
 * MyLinkedList* obj = new MyLinkedList();
 * int param_1 = obj->get(index);
 * obj->addAtHead(val);
 * obj->addAtTail(val);
 * obj->addAtIndex(index,val);
 * obj->deleteAtIndex(index);
 */