707 设计链表（力扣：https://leetcode.cn/problems/design-linked-list/
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struct ListNode{int val;ListNode* prev;ListNode* next;//使用初始化列表ListNode( int num ) : val(num), prev(nullptr), next(nullptr){}
};
//结构体结束的分号不要丢class MyLinkedList {
private:ListNode* head;int size;public:MyLinkedList() {head = nullptr;size = 0;}int get(int index) {if( index >= size){return -1;}ListNode* current = head;for(int i = 0; i < index; i++){current = current->next;}//current是指针，要用->而非.return current->val;}//极端情况是空链表，不要忘记void addAtHead(int val) {//创建值为val的新节点ListNode* newHead = new ListNode(val);if( head = nullptr){head = newHead;}else{//这里只用把新head和旧head的指针更新newHead->next = head;head->prev = newHead;head = newHead;}size++;}//空链表要考虑void addAtTail(int val) {ListNode* newTail = new ListNode(val);if(head = nullptr){head = newTail;}else{ListNode* current = head;//这里用while遍历更高效，因为尾部的特点是next一定为nullptrwhile( current->next != nullptr ){current = current->next;}current->next = newTail;newTail->prev= current;size++;}}void addAtIndex(int index, int val) {//void函数不能breakif(index >= size){return;//头尾用成员函数}else if( index == 0 ){addAtHead(val);}else if( index == size){addAtTail(val);}else{ListNode* current = head;//不需要节点内再定义一个index//for这里按顺序遍历到的i自然对应节点的排位for(int i = 0; i < index - 1; i++){current = current->next;}ListNode* newNode = new ListNode(val);newNode->next = current;current->prev = newNode;if(current->next != nullptr){current->next->prev = newNode;}current->next = newNode;size++;}}void deleteAtIndex(int index) {if (index < 0 || index >= size) {return; // 无效索引}if (index == 0) { // 删除头节点ListNode* toDelete = head;head = head->next;if (head != nullptr) {head->prev = nullptr;}delete toDelete;} else {// 找到第index-1个节点ListNode* current = head;for (int i = 0; i < index - 1; i++) {current = current->next;}ListNode* toDelete = current->next;current->next = toDelete->next;if (toDelete->next != nullptr) {toDelete->next->prev = current;}delete toDelete;}size--;}
};/*** 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);*/