Linked lists are a fundamental data structure in computer science, used to store a collection of elements. They consist of nodes, where each node contains a value and a reference (or link) to the next node in the sequence.

Dynamically resized
No copying overhead

Basics of Linked Lists

Node Structure:

Each node typically contains two parts:

Data: The value stored in the node.
Next: A reference to the next node in the list.

Types of Linked Lists:

Singly Linked List:

Each node points to the next node in the sequence.

Structure

struct Node {
    int data;
    Node* next;
    Node(int val) : data(val), next(nullptr) {}
};

Traversal

void traverse() {
    Node* current = head;
    while (current) {
        cout << current->data;
        current = current->next;
    }
}

Reverse Traversal

// Using stack
void traverseReverse(Node* head) {
    std::stack<int> nodeStack;

    // Traverse the linked list and push data onto the stack
    Node* current = head;
    while (current != nullptr) {
        nodeStack.push(current->data);
        current = current->next;
    }

    // Pop data from the stack to print in reverse order
    while (!nodeStack.empty()) {
        std::cout << nodeStack.top() << " ";
        nodeStack.pop();
    }
    std::cout << std::endl;
}

void traverseReverseRecursive(Node* head) {
    if (head == nullptr) return;

    traverseReverseRecursive(head->next); 

    std::cout << head->data << " "; 
}

Insert

void insert_at_beginning(int data) {
    Node* new_node = new Node(data);
    new_node->next = head;
    head = new_node;
}

void insert_at_middle(int data, int position) {
    if (position == 0) {
        insert_at_beginning(data);
        return;
    }

    Node* new_node = new Node(data);
    Node* current = head;
    for (int i = 0; i < position - 1 && current != nullptr; ++i) {
        current = current->next;
    }

    if (current == nullptr) {
        cout << "Position out of bounds" << endl;
        delete new_node;
        return;
    }

    new_node->next = current->next;
    current->next = new_node;
}

void insert_at_end(int data) {
    Node* new_node = new Node(data);
    if (!head) {
        head = new_node;
        return;
    }

    Node* current = head;
    while (current->next) {
        current = current->next;
    }

    current->next = new_node;
}

Delete

void delete_at_beginning() {
    if (!head) return;

    Node* temp = head;
    head = head->next;
    delete temp;
}

void LinkedList::delete_at_middle(int position) {
    if (!head) return;

    if (position == 0) {
        delete_at_beginning();
        return;
    }

    Node* current = head;
    for (int i = 0; i < position - 1 && current->next != nullptr; ++i) {
        current = current->next;
    }

    if (current->next == nullptr) {
        std::cerr << "Position out of bounds" << std::endl;
        return;
    }

    Node* temp = current->next;
    current->next = current->next->next;
    delete temp;
}


void delete_at_end() {
    if (!head) return;

    if (!head->next) {
        delete head;
        head = nullptr;
        return;
    }

    Node* current = head;
    while (current->next && current->next->next) {
        current = current->next;
    }

    delete current->next;
    current->next = nullptr;
}

Doubly Linked List:
Each node has two references, one to the next node and one to the previous node.

Structure

struct Node {
    int data;
    Node* next;
    Node* prev;
    Node(int val) : data(val), next(nullptr), prev(nullptr) {}
};

Construct DLL

    Node* constructDLL(vector<int>& arr) {
        // code here
        Node* head=new Node(arr[0]);
        Node* temp=head;
        for(int i=1;i<arr.size();i++){
            Node * nn=new Node(arr[i]);
            nn->prev=temp;
            temp->next=nn;
            temp=nn;
        }
        return head;
    }

Insert

void DoublyLinkedList::insert_at_beginning(int data) {
    Node* new_node = new Node(data);
    if (head) {
        new_node->next = head;
        head->prev = new_node;
    }
    head = new_node;
}

void DoublyLinkedList::insert_at_middle(int data, int position) {
    if (position == 0) {
        insert_at_beginning(data);
        return;
    }

    Node* new_node = new Node(data);
    Node* current = head;

    for (int i = 0; i < position - 1 && current != nullptr; ++i) {
        current = current->next;
    }

    if (current == nullptr) {
        cout << "Position out of bounds";
        delete new_node;
        return;
    }

    new_node->next = current->next;
    if (current->next != nullptr) {
        current->next->prev = new_node;
    }
    current->next = new_node;
    new_node->prev = current;
}

void insert_at_end(int data) {
    Node* new_node = new Node(data);
    if (!head) {
        head = new_node;
        return;
    }

    Node* current = head;
    while (current->next) {
        current = current->next;
    }

    current->next = new_node;
    new_node->prev = current;
}

Delete

void delete_at_beginning() {
    if (!head) {
        std::cerr << "List is empty" << std::endl;
        return;
    }
    Node* temp = head;
    head = head->next;
    if (head) {
        head->prev = nullptr;
    }
    delete temp;
}

void DoublyLinkedList::delete_at_middle(int position) {
    if (!head) {
        std::cerr << "List is empty" << std::endl;
        return;
    }
    if (position == 0) {
        delete_at_beginning();
        return;
    }

    Node* current = head;
    for (int i = 0; i < position && current != nullptr; ++i) {
        current = current->next;
    }

    if (current == nullptr) {
        std::cerr << "Position out of bounds" << std::endl;
        return;
    }

    if (current->prev) {
        current->prev->next = current->next;
    }
    if (current->next) {
        current->next->prev = current->prev;
    }
    delete current;
}

void DoublyLinkedList::delete_at_end() {
    if (!head) {
        std::cerr << "List is empty" << std::endl;
        return;
    }

    Node* current = head;
    while (current->next) {
        current = current->next;
    }

    if (current->prev) {
        current->prev->next = nullptr;
    } else {
        head = nullptr; // List had only one node
    }
    delete current;
}

Circular Linked List: The last node points back to the first node, forming a circle.

Important Algorithms

Tortoise Hare

bool detect_cycle() {
    if (!head || !head->next) {
        return false;
    }

    Node* slow = head;
    Node* fast = head;

    while (fast && fast->next) {
        slow = slow->next;        
        fast = fast->next->next

        if (fast == slow) {
            return true;
        }
    }
    return false;
}

Middle of an element

Node* find_middle() {
    if (!head) {
        return nullptr;
    }

    Node* slow = head;
    Node* fast = head;

    while (fast && fast->next) {
        slow = slow->next;
        fast = fast->next->next;
    }

    return slow;
}

Reverse a linkedlist (3 - pointer approach)

    ListNode* reverseList(ListNode* head) {

        ListNode* prev = NULL;
        ListNode* curr = head;

        while(curr != NULL){
            ListNode* forward = curr->next;
            curr->next = prev;
            prev = curr;
            curr = forward;

        }
        return prev;
    }

Merge Sorted Lists

ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
    // Create a dummy node to act as the start of the merged list
    ListNode dummy(0);
    ListNode* tail = &dummy;

    // Traverse both lists
    while (l1 != NULL && l2 != NULL) {
        if (l1->val <= l2->val) {
            tail->next = l1;
            l1 = l1->next;
        } else {
            tail->next = l2;
            l2 = l2->next;
        }
        tail = tail->next;
    }

    // If one of the lists is not empty, append the remaining nodes
    if (l1 != NULL) {
        tail->next = l1;
    } else {
        tail->next = l2;
    }

    return dummy.next;
}

Applications:

Dynamic Memory Management
Implementing Queues and Stacks
Graph Representations
Routing Tables in Networking

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