C-Plus-Plus/data_structures/reverse_a_linked_list.cpp

/**
 * @file
 * @brief Implementation of reversing a single linked list algorithm.
 * @details
 * The linked list is a data structure used for holding a sequence of
 * values, which can be added, displayed,reversed ,removed.
 * ### Algorithm
 * Values can be added by iterating to the end of a list(by following
 * the pointers) starting from the first link. Whichever link points to null
 * is considered the last link and is pointed to the new value.
 *
 * Linked List can be reversed by using 3 pointers ,current ,prev and next_node ;
 * We keep iterating till the last node,meanwhile before changing to the next of current ,we store it in  the next_node pointer;
 * now we store the prev pointer in the current of next ,this is where the actual reversal happens,
 * And then we move the prev and current pointers one step forward.
 * Then the head node is made to point to the last node (prev pointer) after completion of iteration .
 */
#include <iostream>

/**
 * A Node class containing a value and pointer to next node;
 */
class Node
{
public:
    int val;    ///< value of the current link
    Node *next; ///< pointer to the next value on the list
};
/**
 * function adds new element to the end of the list
 * @param n to be added to the end of the list and
 * double pointer to the head node of the list to
 * reflect the changes globally in the whole program
 */
void create(Node **head, int n)
{
    Node *new_node = new Node();
    Node *temp;
    new_node->val = n;
    new_node->next = NULL;
    if (*head == NULL)
    {
        *head = new_node;
    }
    else
    {
        temp = *head;
        while (temp->next != NULL)
            temp = temp->next;
        temp->next = new_node;
    }
}
/**
 * function displays all the elements in the list
 * @returns 'void'
 * @param node pointer for traversing the list.
 */
void printList(Node *node)
{
    while (node != NULL)
    {
        std::cout << node->val << "\t";
        node = node->next;
    }
    std::cout << "\n";
}

/**
 * function reverseList for reversing the  list
 * @param double pointer to head node
 */
void reverseList(Node **head)
{
    Node *curr = *head;
    Node *prev = NULL, *next_node = NULL;
    while (curr != NULL)
    {
        next_node = curr->next;
        curr->next = prev;
        prev = curr;
        curr = next_node;
    }
    *head = prev;
}
/**
 * Main function:
 * Allows the user add ,display and reverse values from the list.
 * @returns 0 on exit
 */
int main()
{
    int n;
    Node *head = NULL;
    while (1)
    {
        std::cout << "\n1. Insert";
        std::cout << "\n2. Display";
        std::cout << "\n3. Resverse";
        std::cout << "\n4. Exit";
        std::cout << "\n\nEnter you choice : ";
        std::cin >> n;

        switch (n)
        {
        case 1:
        {
            int a;
            std::cout << "\nEnter the element to be inserted : ";
            std::cin >> a;
            create(&head, a);

            break;
        }
        case 2:
        {
            std::cout << "Printing the Linked List\n";
            std::cout << "\n";
            printList(head);

            break;
        }
        case 3:
        {
            reverseList(&head);
            break;
        }
        case 4:
        {
            exit(0);
        }
        default:
            std::cout << "Invalid input\n";
        }
    }
    return 0;
}