设计链表（leetcode-707）

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--;
    }
};