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, previous, and
 * next_node; we keep iterating until 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 an iteration.
 * Graphic Explanation:https://bit.ly/3nbVrFe
 */
#include <iostream>/// for I/O operations
#include <memory> /// for dynamic memory
#include <cassert> /// for assert

/**
 * @namespace data_structures
 * @brief Data Structures algorithms
 */
namespace data_structures {

/**
 * @namespace linked_list
 * @brief Functions for singly linked list algorithm
 */
namespace linked_list {
/**
 * A Node class containing a value and pointer to another link
 */
class Node
{
public:
int val;     ///< value of the current link
Node *next;  ///< pointer to the next value on the list
};

/**
 * A list class containing a sequence of links
 */
class list {
 private:
    Node *head{nullptr};  // link before the actual first element
 public:
    /**
     * List constructor. Initializes the first link.
     */
    list() {
        // Initialize the first link
    }

    void insert(int32_t new_elem);
    void reverseList();
    void display();
    int top();
    int last();
};



/**
 * function adds new element to the end of the list
 * @param new_elem to be added to the end of the list
 */
void list::insert(int32_t n) {
   Node *new_node = new Node();
    Node *temp = nullptr;
    new_node->val = n;
    new_node->next = nullptr;
    if (head == nullptr) {
        head = new_node;
    } else {
        temp = head;
        while (temp->next != nullptr) {
            temp = temp->next;
        }
        temp->next = new_node;
    }
}

/**
 * function reverseList for reversing the  list 
 * @brief Using current,previous and next pointer.
 * @returns 'void'
 */
void list:: reverseList() {
    Node *curr = head;
    Node *prev = nullptr, *next_node = nullptr;
    while (curr != nullptr) {
        next_node = curr->next;
        curr->next = prev;
        prev = curr;
        curr = next_node;
    }
    head = prev;
}
/**
 * function to find the top element of the list
 * @returns 'int'
 */
int list::top()
{
    int n=head->val;
    return n;
}
/**
 * function to find the last element of the list
 *  @returns 'int'
 */
int list::last()
{
    Node *t=head;
    while(t->next!=nullptr)
    {
        t=t->next;
    }
    return t->val;
}
/**
 * function displays all the elements in the list
 * @returns 'void'
 */
void list::display() {
    Node *node=head;
    while (node != nullptr) {
        std::cout << node->val << "\t";
        node = node->next;
    }
    std::cout << std::endl;
}


}  // namespace linked_list
}  // namespace data_structures

static void test() {
    data_structures::linked_list::list L;
  // 1st test
   
    L.insert(11);
    L.insert(12);
    L.insert(15);
    L.insert(10);
    L.insert(12);
    L.insert(20);
    L.insert(18);
    assert(L.top()==11);
    assert(L.last()==18);
    L.display(); // To print the array
    L.reverseList();
    //Reversal Testing
    assert(L.top()==18);
    assert(L.last()==11);
    std::cout << "passed" << std::endl;
  
}

/**
 * @brief Main function
 * @returns 0 on exit
 */
int main() {
     test(); // Execute the tests
    return 0;
}