darkan

// Singly Linked List
struct ListNode {
    int val;  // Element stored in the node
    ListNode *next;  // Pointer to the next node
    ListNode(int x) : val(x), next(NULL) {}  // Constructor for the node
};

ListNode* head = new ListNode(5);

class Solution {
public:
    ListNode* removeElements(ListNode* head, int val) {
        ListNode* dummyHead = new ListNode(0); // Set a dummy head node
        dummyHead->next = head; // Point the dummy head to head, making deletion easier
        ListNode* cur = dummyHead;
        while (cur->next != NULL) {
            if(cur->next->val == val) {
                ListNode* tmp = cur->next;
                cur->next = cur->next->next;
                delete tmp;
            } else {
                cur = cur->next;
            }
        }
        head = dummyHead->next;
        delete dummyHead;
        return head;
    }
};

class MyLinkedList {
public:
    // Define the structure of linked list nodes
    struct LinkedNode {
        int val;
        LinkedNode* next;
        LinkedNode(int val):val(val), next(nullptr){}
    };

    // Initialize the linked list
    MyLinkedList() {
        _dummyHead = new LinkedNode(0); // The head node defined here is a dummy head, not the actual head of the linked list
        _size = 0;
    }

    // Get the value of the node at the index, return -1 if index is invalid, note that index starts from 0, the 0th node is the head node
    int get(int index) {
        if (index > (_size - 1) || index < 0) {
            return -1;
        }
        LinkedNode* cur = _dummyHead->next;
        while(index--){ // If --index, it will enter an infinite loop
            cur = cur->next;
        }
        return cur->val;
    }

    // Insert a node at the front of the linked list, after insertion, the newly inserted node becomes the new head of the linked list
    void addAtHead(int val) {
        LinkedNode* newNode = new LinkedNode(val);
        newNode->next = _dummyHead->next;
        _dummyHead->next = newNode;
        _size++;
    }

    // Add a node at the end of the linked list
    void addAtTail(int val) {
        LinkedNode* newNode = new LinkedNode(val);
        LinkedNode* cur = _dummyHead;
        while(cur->next != nullptr){
            cur = cur->next;
        }
        cur->next = newNode;
        _size++;
    }

    // Insert a new node before the node at index, for example, if index is 0, the newly inserted node becomes the new head of the linked list.
    // If index equals the length of the linked list, it means the newly inserted node is the tail of the linked list
    // If index is greater than the length of the linked list, return null
    // If index is less than 0, insert the node at the head
    void addAtIndex(int index, int val) {

        if(index > _size) return;
        if(index < 0) index = 0;        
        LinkedNode* newNode = new LinkedNode(val);
        LinkedNode* cur = _dummyHead;
        while(index--) {
            cur = cur->next;
        }
        newNode->next = cur->next;
        cur->next = newNode;
        _size++;
    }

    // Delete the node at index, if index is greater than or equal to the length of the linked list, return directly, note that index starts from 0
    void deleteAtIndex(int index) {
        if (index >= _size || index < 0) {
            return;
        }
        LinkedNode* cur = _dummyHead;
        while(index--) {
            cur = cur ->next;
        }
        LinkedNode* tmp = cur->next;
        cur->next = cur->next->next;
        delete tmp;
        // The delete command indicates that the memory originally pointed to by the tmp pointer has been released,
        // the value (address) of the pointer tmp after being deleted is not NULL, but a random value. That is, after being deleted,
        // if you do not add a statement tmp=nullptr, tmp will become a dangling pointer
        // If the subsequent program accidentally uses tmp, it will point to unpredictable memory space
        tmp=nullptr;
        _size--;
    }

    // Print the linked list
    void printLinkedList() {
        LinkedNode* cur = _dummyHead;
        while (cur->next != nullptr) {
            cout << cur->next->val << " ";
            cur = cur->next;
        }
        cout << endl;
    }
private:
    int _size;
    LinkedNode* _dummyHead;

};

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* temp; // Save the next node of cur
        ListNode* cur = head;
        ListNode* pre = NULL;
        while(cur) {
            temp = cur->next;  // Save cur's next node, because we will change cur->next next
            cur->next = pre; // Reversal operation
            // Update pre and cur pointers
            pre = cur;
            cur = temp;
        }
        return pre;
    }
};

class Solution {
public:
    ListNode* reverse(ListNode* pre,ListNode* cur){
        if(cur == NULL) return pre;
        ListNode* temp = cur->next;
        cur->next = pre;
        // This can be compared with the two-pointer method code, the recursive writing actually does these two steps
        // pre = cur;
        // cur = temp;
        return reverse(cur,temp);
    }
    ListNode* reverseList(ListNode* head) {
        // The initialization logic is the same as the two-pointer method
        // ListNode* cur = head;
        // ListNode* pre = NULL;
        return reverse(NULL, head);
    }

};