思路：
（1）获取第n个结点的值
明确链表结点下标从0开始，第n个结点
（2）头部插入结点
注意插入的顺序
（3）尾部插入结点
（4）第n个结点前插入结点
（5）删除第n个结点
class MyLinkedList {struct NodeList {int val;NodeList* next;NodeList(int val):val(val), next(nullptr){}};private:int size;NodeList* dummyhead;public:MyLinkedList() {dummyhead = new NodeList(0); size = 0;}int get(int index) {if (index < 0 || index > (size - 1)) {return -1;}NodeList* current = dummyhead->next;while(index--){ current = current->next;}return current->val;}void addAtHead(int val) {NodeList* Newnode = new NodeList(val);Newnode->next = dummyhead->next;dummyhead->next = Newnode;size++;}void addAtTail(int val) {NodeList* Newnode = new NodeList(val);NodeList* current = dummyhead;while(current->next != NULL){current = current->next;}current->next = Newnode;size++;}void addAtIndex(int index, int val) {if (index > size) {return ;}NodeList* Newnode = new NodeList(val);NodeList* current = dummyhead;while(index--) {current = current->next;}Newnode->next = current->next;current->next = Newnode;size++;}void deleteAtIndex(int index) {if (index >= size || index < 0) {return ;}NodeList* current = dummyhead;while(index--) {current = current ->next;}NodeList* tmp = current->next;current->next = current->next->next;delete tmp;size--;}
};