# Design Linked List

## [Design Linked List](https://leetcode.com/problems/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:**

```

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
```

**Constraints:**

* `0 <= index, val <= 1000`
* Please do not use the built-in LinkedList library.
* At most `2000` calls will be made to `get`, `addAtHead`, `addAtTail`,  `addAtIndex` and `deleteAtIndex`.

## Solutions

### 🧠 Cpp

```cpp
struct Node
{
    Node(int v) : val(v){}
    int val;
    Node *next = nullptr;
};

class MyLinkedList
{
    Node *head = nullptr,
         *tail = nullptr;
    size_t size = 0;

public:
    /** Initialize your data structure here. */
    MyLinkedList() = default;

    /** Get the value of the index-th node in the linked list. If the index is invalid, return -1. */
    int get(int index)
    {
        if(!size || index > size-1) 
            return -1;

        Node *asked_node = head;
        //node at index
        for(; index != 0; --index)
            asked_node = asked_node->next;

        return asked_node->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 addAtHead(int val)
    {
        Node *new_node = new Node(val);

        //init
        if(!head)
            head = tail = new_node;
        else
        {
            new_node->next = head;
            head = new_node;
        }
        size++;
    }

    /** Append a node of value val to the last element of the linked list. */
    void addAtTail(int val)
    {
        Node *new_node = new Node(val);
        //init
        if(!head)
            head = tail = new_node;
        else
        {
            tail->next = new_node;
            tail = new_node;
        }
        size++;
    }

    /** Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted. */
    void addAtIndex(int index, int val)
    {
        //index should exist
        // or index == size, it`s push_back
        if(index > size) 
            return;


        if(index == 0)
            addAtHead(val);
        else if(index == size)
            addAtTail(val);
        else
        {
            Node *new_node = new Node(val),
                 //one node before insertion position
                 *prev_node = head;
            for(; index != 1; --index)
                prev_node = prev_node->next;

            new_node->next = prev_node->next;
            prev_node->next = new_node;
            size++;    
        }

    }

    /** Delete the index-th node in the linked list, if the index is valid. */
    void deleteAtIndex(int index)
    {
        //index should exist
        if(index > size-1) 
            return;


        if(index == 0)
        {
            //delete head
            Node *head_next = head->next;
            if(head == tail)
                tail = nullptr;

            delete head;
            head = head_next;
        }
        else if(index == size-1)
        {
            //delete tail
            Node *prev_node = head;
            //get node before index
            for(; index != 1; --index)
                prev_node = prev_node->next;

            prev_node->next = nullptr;
            delete tail;
            tail = prev_node;
        }
        else
        {
            //delete node in the middle
            Node *prev_node = head;
            //get node before index
            for(; index != 1; --index)
                prev_node = prev_node->next;

            Node *to_delete = prev_node->next;
            prev_node->next = to_delete->next;
            delete to_delete;

        }
        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);
 */
```


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