Doubly Linked List [implemented on singly]

#include <iostream>
#include <stdlib.h>
using namespace std;

struct node {
    int data;
    node *next, *prev;
} *head, *tail, *newNODE;

int length;

void listInitiate(); //
void create_List(int); //
void show_List();
void show_List2(); //
void search_List(int);
void search_List2(int); //
void insert_AT_first(int); //
void insert_after_value(int,int); //
void insert_before_value(int,int);
void insert_between_values(int,int,int);
void insert_AT_pos(int, int);
void insert_AT_last(int); //
void delete_AT_first(); //
void delete_after_value(int); //
void delete_before_value(int);
void delete_between_values(int, int);
void delete_AT_pos(int);
void delete_AT_last(); //

int main() {
    create_List(4);
    show_List();
    delete_after_value(3);
    show_List();
    show_List2();
}

void listInitiate() {
    head = NULL;
    tail = NULL;
}

void create_List(int n) {
    listInitiate();
    cout << "\nCreating a new list of size " << n << "." << endl; length = n;
    for (int i = 0; i < n; i++) {
        int item; cout << "\nElement " << i + 1 << ": "; cin >> item;
        newNODE = new node;
        newNODE->data = item;
        newNODE->prev = tail;
        newNODE->next = NULL;
        if (head == NULL) {
            head = newNODE;
            tail = newNODE;
        }
        else {
            tail->next = newNODE;
            tail = newNODE;
        }
    }
}

void show_List() { node *i; int l;
    if (head != NULL) {
        cout << "\nCurrently in the List, you have following elements: \n";
        for (i = head, l = 1; i != NULL; i = i->next, l++) {
            cout << "\nElement " << l << ": " << i->data << endl;
        }
        cout << "\nEnd of the list." << endl;
    }
    else cout << "\nYour list is empty." << endl;
}

void show_List2() {
    node *i; int l;
    if (head != NULL) {
        cout << "\nReversed list : " << endl;
        for (i = tail, l = 1; i != NULL; i = i->prev, l++) {
            cout << "\nElement " << l << ": " << i->data << endl;
        }
        cout << "\nEnd of the list." << endl;
    }
    else cout << "\nYour list is empty." << endl;
}

void search_List(int item) { node *i; int l;
    for (i = head, l = 1; i->data != item && i->next != NULL; l++)
        i = i->next;
    if (i->data == item)
        cout << "\nElement found..! Position of first appearance in the list : " << l << endl;
    else cout << "\nSorry! Element isn't in the list." << endl;
}

void search_List2(int item) {
    node *i; int l;
    for (i = tail, l = 1; i->data != item && i->prev != NULL; l++)
        i = i->prev;
    if (i->data == item)
        cout << "\nElement found..! Position of first appearance in the reversed list : " << l << endl;
    else cout << "\nSorry! Element isn't in the list." << endl;
}

void insert_AT_first(int item) {
    newNODE = new node;
    newNODE->data = item;
    newNODE->next = head;
    newNODE->prev = NULL;
    head->prev = newNODE;
    head = newNODE;
    cout << "\nElement " << item << " has been added as the new head of the list." << endl;
}

void insert_after_value(int value, int item) {
    node *i; int l;
    for (i = head, l = 1; i->data != value && i->next != NULL; l++)
        i = i->next;
    if (i->data == value) {
        newNODE = new node;
        newNODE->data = item;
        newNODE->next = i->next;
        newNODE->prev = i;
        newNODE->next->prev = newNODE;
        i->next = newNODE;
        cout << "\nElement " << value << " has been found on the list at postion no: " << l
            << " and " << item << " has been added after " << value << " at position no: "
            << l + 1 << endl;
    }
    else {
        cout << "\nOPERATION failed..!" << endl;
        cout << "Searched for " << value << " among the " << l << " elements of the list but "
            << value << " could not be found." << endl;
    }
}

void insert_before_value(int value, int item) {
    node *i; int l;
    if (head->data == value) {
        insert_AT_first(item);
        cout << "\nElement " << value <<
            " has been found on the head so " << item << " has been added as the new head."
            << endl;
    }
    else {
        for (i = head, l = 1; i->next->data != value && i->next != NULL; l++)
            i = i->next;
        if (i->next->data == value) {
            newNODE = new node;
            newNODE->data = item;
            newNODE->next = i->next;
            i->next = newNODE;
            cout << "\nElement " << value << " has been found on the list at postion no: " << l + 1
                << " and " << item << " has been added before " << value << " at position no: "
                << l + 1 << endl;
        }
        else {
            cout << "\nOPERATION failed..!" << endl;
            cout << "Searched for " << value << " among the " << l << " elements of the list but "
                << value << " could not be found." << endl;
        }
    }
}

void insert_between_values(int val1, int val2, int item) {
    node *i; int l;
    for (i = head, l = 1; i->data != val1 && i->next != NULL; l++)
        i = i->next;
    if (i->data == val1 && i->next->data == val2) {
        newNODE = new node;
        newNODE->data = item;
        newNODE->next = i->next;
        i->next = newNODE;
        cout << "\nElement " << val1 << " has been found on the list at postion no: " << l
            << " and element " << val2 << " has been found on the list at positon no: " << l + 1
            << " So, " << item << " has been added between them at position no: " << l + 1 << endl;
    }
    else {
        cout << "\nOPERATION failed..!" << endl;
        cout << val1 << " and " << val2 << " couldn't be found sequencially." << endl;
    }
}

void insert_AT_pos(int position, int item) {
    node *i; int l;
    if (position == 1) {
        insert_AT_first(item);
    }
    else {
        for (i = head, l = 1; l != position - 1 && i->next != NULL; l++)
            i = i->next;
        if (l == position - 1) {
            newNODE = new node;
            newNODE->data = item;
            newNODE->next = i->next;
            i->next = newNODE;
            cout << "\nElement " << item << " has been added at the list at position no: "
                << l + 1 << endl;
        }
        else {
            cout << "\nOPERATION failed..!" << endl;
            cout << "\nGiven position is out of the range." << endl;
        }
    }
}

void insert_AT_last(int item) {
    node *i; int l;
    newNODE = new node;
    newNODE->data = item;
    newNODE->next = NULL;
    newNODE->prev = tail;
    if (head == NULL) {
        head = newNODE;
        tail = newNODE;
        cout << "\nList was empty so, data " << item << " has been added at the list as the head."
            << endl;
    }
    else {
        tail->next = newNODE;
        tail = tail->next;
        cout << "\nElement " << item << " has been added at the tail of the list at position no: "
            << ++length << endl;
    }
}

void delete_AT_first() {
    node *i = head;
    head = head->next;
    head->prev = NULL;
    cout << "\nPrevious head " << i->data << " has been deleted." << endl;
    delete i;
    if (head != NULL)
        cout << "\nNew head value: " << head->data << endl;
}

void delete_after_value(int value) {
    node *i; int l;
    for (i = head, l = 1; i->data != value && i->next != NULL; l++)
        i = i->next;
    if (i->data == value && i->next != NULL) {
        newNODE = new node;
        newNODE = i->next;
        i->next = i->next->next;
        if (i->next != NULL) {
            i->next->prev = i;
        }
        else tail = i;
        newNODE->next = NULL;
        newNODE->prev = NULL;
        delete newNODE;
        cout << "\nElement " << value << " has been found at position no: " << l << " so, item at"
            << " position no: " << l + 1 << " has been deleted." << endl;
    }
    else if (i->data == value && i->next == NULL) {
        cout << "\nElement found at position no: " << l << ", which is the last element of the list."
            << " So, operation could not be done." << endl;
    }
    else {
        cout << "\nOPERATION Failed..!" << endl;
        cout << "\nGiven Value isn't in the list." << endl;
    }
}

void delete_before_value(int value) {
    if (head->data == value) {
        cout << "\nOperation Failed. No element exists before the head." << endl;
    }
    else if (head->next != NULL) {
        if (head->next->data == value)
            delete_AT_first();
        else if (head->next->next != NULL) {
            node *i; int l;
            for (i = head, l = 3; i->next->next->data != value && i->next->next->next != NULL; l++)
                i = i->next;
            if (i->next->next->data == value) {
                newNODE = new node;
                newNODE = i->next;
                i->next = newNODE->next;
                newNODE->next = NULL;
                delete newNODE;
                cout << "\nElement " << value << " has been found at position no: " << l << " therefore,"
                    << " element at position no: " << l - 1 << " has been deleted." << endl;
            }
            else cout << "\nOperation Failed. Such element doesn't exist on the list." << endl;
        }
        else cout << "\nOperation Failed. Such element doesn't exist on the list." << endl;
    }
    else cout << "\nOperation Failed. Such element doesn't exist on the list." << endl;
}

void delete_between_values(int val1, int val2) {
    node *i; int l;
    for (i = head, l = 1; i->data != val1 && i->next != NULL; l++)
        i = i->next;
    if (i->data == val1 && i->next != NULL) {
        if (i->next->next != NULL && i->next->next->data == val2) {
            newNODE = new node;
            newNODE = i->next;
            i->next = i->next->next;
            newNODE->next = NULL;
            cout << "\nElement " << val1 << " has been found at position no: " << l << " and element "
                << val2 << " has been found at position no: " << l + 2 << " therefore, data "
                << newNODE->data << " has been deleted from position no: " << l + 1 << endl;
        }
        else cout << "\nNo such element exists between " << val1 << " and " << val2
            << " or may be " << val2 << " doesn't exist." << endl;
    }
    else cout << "\nEither " << val1 << " or " << val2 << " or both doesn't exist." << endl;
}

void delete_AT_pos(int position) {
    node *i; int l;
    if (position == 1)
        delete_AT_first();
    else {
        for (i = head, l = 1; l != position - 1 && i->next != NULL; l++)
            i = i->next;
        if (i->next == NULL)
            cout << "\nInvalid position." << endl;
        else {
            newNODE = new node;
            newNODE = i->next;
            i->next = newNODE->next;
            newNODE->next = NULL;
            cout << "\nElement at position no: " << position << " is " << newNODE->data << " and just got"
                << " deleted." << endl;
            delete newNODE;
        }
    }
}

void delete_AT_last() {
    node *i;
    if (head->next == NULL)
        delete_AT_first();
    else {
        newNODE = new node;
        newNODE = tail;
        tail = tail->prev;
        tail->next = NULL;
        newNODE->prev = NULL;
        cout << "\nLast element of the list was " << newNODE->data << " which was at position no: "
            << length-- << " and just got deleted." << endl;
        delete newNODE;
    }
}