Swap Nodes of Linked List without Copying

C++ Code:

void swap_nodes(Node **headPtr, int a, int b){

    if(a == b || !headPtr || !(*headPtr))

        return;

    

    Node *curr = *headPtr;

    Node *prev = nullptr;

    Node *nodeA = nullptr;

    Node *nodeB = nullptr;

    Node *prevA = nullptr;

    Node *prevB = nullptr;

    

    while (curr) {

        if(curr->data == a){

            nodeA = curr;

            prevA = prev;

        } else if(curr->data == b){

            nodeB = curr;

            prevB = prev;

        }

        prev = curr;

        curr = curr->next;

    }

    

    if(!nodeA || !nodeB)

        return;

    

    if(prevA){

        prevA->next = nodeB;

    } else { // nodeA is head

        *headPtr = nodeB;

    }

    

    if(prevB){

        prevB->next = nodeA;

    } else { // nodeB is head

        *headPtr = nodeA;

    }

    

    Node *tmp = nodeA->next;

    nodeA->next = nodeB->next;

    nodeB->next = tmp;

}

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Check if a Linked List is palindrome

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Remove Loop From Linked List using Floyd's Algorithm

C++ Code:

void remove_loop(Node *head){

    Node *slow = head;

    Node *fast = head;

    if (!head || !(head->next))

        return;

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

        slow = slow->next;

        fast = fast->next->next;

        if(slow == fast)

            break;

    }

    

    // No Loop exixts

    if (slow != fast){

        std::cout << "No Loop" << std::endl;

        return;

    }

    

    // When Loop exists

    slow = head;

    while(slow->next != fast->next){

        slow = slow->next;

        fast = fast->next;

    }

    fast->next = nullptr;

}

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Why Floyd's Loop Detection Algorithm works ?

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Find First Element of Loop in Linked List

C++ Code:

void find_loop_beginning(Node *head){

    Node *slow = head;

    Node *fast = head;

    if (!head || !(head->next))

        return;

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

        slow = slow->next;

        fast = fast->next->next;

        if(slow == fast)

            break;

    }

    

    // No Loop exixts

    if (slow != fast){

        std::cout << "No Loop" << std::endl;

        return;

    }

    

    // When Loop exists

    slow = head;

    while(slow != fast){

        slow = slow->next;

        fast = fast->next;

    }

    std::cout << "First loop element = " << slow->data << std::endl;

}

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Find k-th node from end in Linked List

void print_kth_from_end(Node *head, int k){

    Node *lead = head;

    Node *lag = head;

    while(k > 0){

        if(lead){

            k--;

            lead = lead->next;

        }

        else{

            std::cout << "K is larger than length of Linked List" << std::endl;

            return;

        }

    }

    while(lead){

        lead = lead->next;

        lag = lag->next;

    }

    std::cout << "kth node from end = " << lag->data << std::endl;

}

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Find the middle element of a Linked List

void print_middle(Node *head){

    Node *fast = head;

    Node *slow = head;

    while(fast && fast->next){

        fast = fast->next->next;

        slow = slow->next;

    }

    std::cout << slow->data << std::endl;

}

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