10_linked_list_recursive_reverse.cpp

/*
    TOPIC: Recursive Reverse a Linked List

    NOTE: We will be swapping the POINTER not the DATA to reverse a linked list.
          As, Swapping data of nodes may be expensive in many situations when data contains many fields. 
*/


#include <iostream>
using namespace std;

class Node{
public:
    int data;
    Node *next;

    // constructor
    Node(int d)
    {
        data = d;
        next = NULL;
    }
};


// pass a pointer by reference (because we want to make changes to the original head pointer)
void insertAtHead(Node *&head, int d)
{
    if(head == NULL)
    {
        head = new Node(d);
        return;
    }

    Node *n = new Node(d);
    n->next = head;
    head = n;
}



// function to find length of linked list
int length(Node*head)
{
    int cnt = 0;
    while(head != NULL)
    {
        cnt++;
        head = head->next;
    }
    return cnt;
}



// function to insert data at last position of linked list
void insertAtTail(Node*&head, int data)
{
    // corner case
    if(head == NULL)
    {
        // Node *n =  new Node(data);
        // head = n;
        head = new Node(data);
        return;
    }
    Node *tail = head;             // using temp pointer
    // Moving head towards tail
    while(tail->next != NULL)
    {
        tail = tail->next;
    }
    // create & attach new node
    tail->next = new Node(data);
}



// function to insert data at middle of linked list
void insertAtMiddle(Node*&head, int data, int pos)
{
    //corner case
    if(head==NULL or pos==0)
    {
        insertAtHead(head, data);
        return;
    }
    else if (pos >= length(head))
    {
        insertAtTail(head, data);
        return;
    }
    // take pos-1 jumps
    int i=1;
    Node *temp = head;
    while(i < pos)
    {
        temp = temp->next;
        i++;
    }
    // create a new node
    Node *n = new Node(data);
    n->next = temp->next;
    temp->next = n;
}

// pass by value (because we don't want to change the original head pointer)
void print(Node *head)
{
    while(head != NULL)
    {
        cout << head->data << " -> ";
        head = head->next;
    }
    cout << endl;
}



// function to delete data at start of linked list
void deleteAtHead(Node*&head)
{
    if(head == NULL)
    {
        return;
    }
    Node*temp = head->next;   // temp pointer is static. So it will be deleted at the end of function call 
    delete head;              // delete the Node whose address is stored in "head" pointer
    head = temp;
}



// function to delete data at end of linked list
void deleteAtTail(Node*&head)
{
    if(head == NULL)
    {
        return;
    }
    Node *tail = head;
    Node *prev;
    // moving towards the end of linked list
    while(tail->next != NULL)
    {
        prev = tail;
        tail = tail->next;
    }
    delete tail;                  // delete the Node whose address is stored in "tail" pointer
    // delete prev->next;         // prev->nex & tail are pointing towards same address
    prev->next = NULL;
}



// function to delete data at end of linked list
void deleteAtMiddle(Node*&head, int pos)
{
    if(head==NULL)
    {
        return;
    }
    else if(pos >= length(head))
    {
        return;
    }
    // jump towards pos
    Node *temp = head;
    Node *prev;
    int i = 1;
    while(i <= pos)
    {
        prev = temp;
        temp = temp->next;
        i++;
    }
    prev->next = temp->next;
    delete temp;              // delete the Node whose address is stored in "temp" pointer
}


// Search Operation
// Linear Search
bool search(Node*head, int key)
{
    Node*temp = head;
    while(temp != NULL)
    {
        if(temp->data == key)
        {
            return true;
        }
        temp = temp->next;
    }
    // if key not found
    return false;
}


// Linear Search (recursively)
bool searchRecursive(Node*head, int key)
{
    //base case
    if(head == NULL)
    { 
        return false;
    }
    //rec case
    if(head->data == key)
    {
        return true;
    }
    return searchRecursive(head->next, key);
}


// Linked List User Input
void take_input(Node*&head)
{
    //
}

// Linked List User Input
Node* take_input_2()
{
    Node *head = NULL;

    int data;
    cin >> data;

    while(data != -1)
    {
        insertAtHead(head, data);
        // insertAtTail(head, data);
        cin >> data;
    }
    return head;
    /* NOTE: Since we are adding elements at "head" of linked list.
             So, we will be getting linked list in the reverse order.
    */
}


// Linked List User Input [When taking Input from a file]
Node* take_input_file()
{
    Node *head = NULL;

    int data;
    // cin >> data;

    while(cin >> data)
    // while(data != -1)
    {
        insertAtHead(head, data);
        // cin >> data;
    }
    return head;
    /* NOTE: Since we are adding elements at "head" of linked list.
             So, we will be getting linked list in the reverse order.
    */
}


/* 
    - Using "<<" for getting output
      Eg: cout << head;   
          This will print the address of head.
          But we want to print entire linked list uing "cout". 
          So, we will use operator overloading.
      
      NOTE:    cout << head;
               |         |
            ostream    Node
             object    object

         So, "<<" operator will take 2 parameter.
              One is Ostream object (for "cout")
              Second is Node object (for "head")
*/


// // Opertor Overloading (i.e for cases like: cout << head;)
// void operator<<(ostream &os, Node *head)
// {
//     print(head);
//     return;
// }


// Cascading of Operators (i.e for cases like: cout << head << head2;)
ostream& operator<<(ostream &os, Node *head)   // Both Return & Pass are by Reference
{
    print(head);
    return os;     // returning "cout" by reference

    /* 
        => cout << head << head2;
          |-----------|
           return cout

        =>        cout << head2;
                  |------------|
                    return cout

        =>               cout;  [OUTPUT]
        
        NOTE: We will get "cout; as output.
              "cout;" is a valid statement in C++ & writing it doesnot do anything.
    */
}


// Cascading of Operators (i.e for cases like: cin >> head >> head2;)
istream& operator>>(istream &is, Node *&head)
{
    head = take_input_2();
    return is;
}


// reverse a linked list  [Complexity: O(N) time + O(1) space]
void reverse(Node *&head)
{
    Node *curr = head;  // for current node
    Node *prev = NULL;  // for previous node
    Node *nxt;          // for next node

    while(curr != NULL)
    {
        nxt = curr->next;     // save the next node
        curr->next = prev;    // make the current node point to previous node
        prev = curr;          // update previous node (for the next iteration)
        curr = nxt;           // update current node (for the next iteration)
    }

    head = prev;              // update head
}


// recursively reverse a linked list  [Complexity: O(N) time + O(N) space]
Node* recursive_reverse(Node *head)
{
    // smallest linked list or when actual linked list is null
    if(head->next == NULL or head == NULL)
    {
        return head;
    }
    // rec case
    Node *smallhead = recursive_reverse(head->next);

    // Node *temp = smallhead;        // Before  Optimization    [Complexity: O(N^2) time] i.e O(N) for both recursion & Loop 
    // while(temp->next != NULL)
    // {
    //     temp = temp->next;
    // } 
    Node *temp = head->next;          // After Optimization      [Complexity: O(N) time]

    temp->next = head;
    head->next = NULL;
    return smallhead;

    /*  
    // NOTE: This is also correct.

    Node *smallhead = recursive_reverse(head->next);

    head->next->next = head;

    head->next = NULL;
    return smallhead;
     */
}



int main()
{
    Node *head = NULL;

    cout << "Enter Elements [Press -1 to Exit]: ";
    cin >> head;

    cout << "Linked List: ";
    // print(head);
    cout << head;

    head = recursive_reverse(head);   // recursively reverse the linked list

    cout << "Linked List [After Reversed]: ";
    cout << head;

    return 0;
}


/* 
OUTPUT:

    Enter Elements [Press -1 to Exit] : 1 2 3 4 5 -1
    Linked List                       : 5 -> 4 -> 3 -> 2 -> 1
    Linked List [After Reversed]      : 1 -> 2 -> 3 -> 4 -> 5
*/