707. Design Linked List

Design your implementation of the linked list. You can choose to use a singly or doubly linked list.
A node in a singly linked list should have two attributes: val and next. val is the value of the current node, and next is a pointer/reference to the next node.
If you want to use the doubly linked list, you will need one more attribute prev to indicate the previous node in the linked list. Assume all nodes in the linked list are 0-indexed.

Implement the MyLinkedList class:

  • MyLinkedList() Initializes the MyLinkedList object.
  • int get(int index) Get the value of the indexth node in the linked list. If the index is invalid, return -1.
  • void addAtHead(int val) Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list.
  • void addAtTail(int val) Append a node of value val as the last element of the linked list.
  • void addAtIndex(int index, int val) Add a node of value val before the indexth node in the linked list. If index equals the length of the linked list, the node will be appended to the end of the linked list. If index is greater than the length, the node will not be inserted.
  • void deleteAtIndex(int index) Delete the indexth node in the linked list, if the index is valid.

Example 1:

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Input
["MyLinkedList", "addAtHead", "addAtTail", "addAtIndex", "get", "deleteAtIndex", "get"]
[[], [1], [3], [1, 2], [1], [1], [1]]
Output
[null, null, null, null, 2, null, 3]

Explanation
MyLinkedList myLinkedList = new MyLinkedList();
myLinkedList.addAtHead(1);
myLinkedList.addAtTail(3);
myLinkedList.addAtIndex(1, 2);    // linked list becomes 1->2->3
myLinkedList.get(1);              // return 2
myLinkedList.deleteAtIndex(1);    // now the linked list is 1->3
myLinkedList.get(1);              // return 3

题目大意

设计一个单链表或双链表的实现，支持以下操作：

  • 初始化链表
  • 获取指定索引节点的值
  • 在链表头部添加节点
  • 在链表尾部添加节点
  • 在指定索引前插入节点
  • 删除指定索引的节点

解题思路

我们可以使用单链表来实现，通过定义节点结构和一个头指针，以及一个记录链表长度的变量来简化操作：

  1. 定义节点结构，包含值和指向下一个节点的指针
  2. 使用虚拟头节点（dummy head）简化边界条件处理
  3. 维护一个长度变量，方便快速判断索引是否有效
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class MyLinkedList {
private:
    // 定义节点结构
    struct ListNode {
        int val;
        ListNode* next;
        ListNode(int x) : val(x), next(nullptr) {}
    };
    
    ListNode* dummyHead;  // 虚拟头节点
    int length;           // 链表长度
    
public:
    // 初始化链表
    MyLinkedList() {
        dummyHead = new ListNode(0);
        length = 0;
    }
    
    // 获取索引为index的节点值
    int get(int index) {
        // 索引无效的情况
        if (index < 0 || index >= length) {
            return -1;
        }
        
        ListNode* curr = dummyHead->next;
        for (int i = 0; i < index; ++i) {
            curr = curr->next;
        }
        return curr->val;
    }
    
    // 在头部添加节点
    void addAtHead(int val) {
        ListNode* newNode = new ListNode(val);
        newNode->next = dummyHead->next;
        dummyHead->next = newNode;
        length++;
    }
    
    // 在尾部添加节点
    void addAtTail(int val) {
        ListNode* newNode = new ListNode(val);
        ListNode* curr = dummyHead;
        
        // 找到最后一个节点
        while (curr->next != nullptr) {
            curr = curr->next;
        }
        
        curr->next = newNode;
        length++;
    }
    
    // 在索引为index的节点前插入新节点
    void addAtIndex(int index, int val) {
        // 索引大于长度，不插入
        if (index > length) {
            return;
        }
        // 索引小于0，插入到头部
        if (index < 0) {
            index = 0;
        }
        
        ListNode* newNode = new ListNode(val);
        ListNode* curr = dummyHead;
        
        // 找到要插入位置的前一个节点
        for (int i = 0; i < index; ++i) {
            curr = curr->next;
        }
        
        newNode->next = curr->next;
        curr->next = newNode;
        length++;
    }
    
    // 删除索引为index的节点
    void deleteAtIndex(int index) {
        // 索引无效，不删除
        if (index < 0 || index >= length) {
            return;
        }
        
        ListNode* curr = dummyHead;
        
        // 找到要删除节点的前一个节点
        for (int i = 0; i < index; ++i) {
            curr = curr->next;
        }
        
        ListNode* temp = curr->next;
        curr->next = curr->next->next;
        delete temp;  // 释放内存，避免泄漏
        length--;
    }
};

/**
 * 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);
 */