Linked list reversal

// In place reversal linked list

using namespace std;

#include <iostream>

class ListNode {
 public:
  int value = 0;
  ListNode *next;

  ListNode(int value) {
    this->value = value;
    next = nullptr;
  }
};

class ReverseLinkedList {
 public:
  static ListNode *reverse(ListNode *head) {
    if (!head) {
      return head;
    }
    ListNode *current = head;
    ListNode *prevPtr = nullptr;
    ListNode *nextPtr = nullptr;
    while (current) {
      nextPtr = current->next;
      current->next = prevPtr;
      prevPtr = current;
      current = nextPtr;
    }
    return prevPtr;
  }
};

int main(int argc, char *argv[]) {
  ListNode *head = new ListNode(2);
  head->next = new ListNode(4);
  head->next->next = new ListNode(6);
  head->next->next->next = new ListNode(8);
  head->next->next->next->next = new ListNode(10);

  ListNode *result = ReverseLinkedList::reverse(head);
  cout << "Nodes of the reversed LinkedList are: ";
  while (result != nullptr) {
    cout << result->value << " ";
    result = result->next;
  }
}


// Reverse sublist of linkedList
using namespace std;

#include <iostream>

class ListNode {
 public:
  int value = 0;
  ListNode *next;

  ListNode(int value) {
    this->value = value;
    next = nullptr;
  }
};

class ReverseSubList {
 public:
  static ListNode *reverse(ListNode *head, int p, int q) {
    if (p == q) {
      return head;
    }

    // after skipping 'p-1' nodes, current will point to 'p'th node
    ListNode *current = head, *previous = nullptr;
    for (int i = 0; current != nullptr && i < p - 1; ++i) {
      previous = current;
      current = current->next;
    }

    // we are interested in three parts of the LinkedList, part before index 'p', part between 'p'
    // and 'q', and the part after index 'q'
    ListNode *lastNodeOfFirstPart = previous;  // points to the node at index 'p-1'

    // after reversing the LinkedList 'current' will become the last node of the sub-list
    ListNode *lastNodeOfSubList = current;
    ListNode *next = nullptr;  // will be used to temporarily store the next node

    // reverse nodes between 'p' and 'q'
    for (int i = 0; current != nullptr && i < q - p + 1; i++) {
      next = current->next;
      current->next = previous;
      previous = current;
      current = next;
    }

    // connect with the first part
    if (lastNodeOfFirstPart != nullptr) {
      lastNodeOfFirstPart->next = previous;  // 'previous' is now the first node of the sub-list
    } else {  // this means p == 1 i.e., we are changing the first node (head) of the LinkedList
      head = previous;
    }

    // connect with the last part
    lastNodeOfSubList->next = current;

    return head;
  }
};

int main(int argc, char *argv[]) {
  ListNode *head = new ListNode(1);
  head->next = new ListNode(2);
  head->next->next = new ListNode(3);
  head->next->next->next = new ListNode(4);
  head->next->next->next->next = new ListNode(5);

  ListNode *result = ReverseSubList::reverse(head, 2, 4);
  cout << "Nodes of the reversed LinkedList are: ";
  while (result != nullptr) {
    cout << result->value << " ";
    result = result->next;
  }
}