C++ - Reverse the Circular Singly Linked List
While working with a circular singly linked list, sometimes it is required to reverse it. Reversing a List produces following result: if the given List is 10->20->30->40->50, after reversing the List the List becomes 50->40->30->20->10.
Reversing a List requires creating three nodes, considering that the list is not empty, which are as follows: tempNode pointing to head, prevNode pointing to head and curNode pointing to next of head. Then make next of prevNode as itself to make the first node as last node of the reversed list. After that, while the curNode is not head adjust links (unlink curNode and link it to the reversed list from front and modify curNode and prevNode to the next element in the list). At last, make the prevNode (last node) as head.
The function reverseList is created for this purpose. It is a 4-step process.
void reverseList() { //1. If head is not null create three nodes // prevNode - pointing to head, // tempNode - pointing to head, // curNode - pointing to next of head if(head != NULL) { Node* prevNode = head; Node* tempNode = head; Node* curNode = head->next; //2. assign next of prevNode as itself to make the // first node as last node of the reversed list prevNode->next = prevNode; while(curNode != head) { //3. While the curNode is not head adjust links // (unlink curNode and link it to the reversed list // from front and modify curNode and prevNode) tempNode = curNode->next; curNode->next = prevNode; head->next = curNode; prevNode = curNode; curNode = tempNode; } //4. Make prevNode (last node) as head head = prevNode; } }
The below is a complete program that uses above discussed concept to reverse a given circular singly linked list.
#include <iostream> using namespace std; //node structure struct Node { int data; Node* next; }; class LinkedList { private: Node* head; public: LinkedList(){ head = NULL; } //Add new element at the end of the list void push_back(int newElement) { Node* newNode = new Node(); newNode->data = newElement; newNode->next = NULL; if(head == NULL) { head = newNode; newNode->next = head; } else { Node* temp = head; while(temp->next != head) temp = temp->next; temp->next = newNode; newNode->next = head; } } //reverse the list void reverseList() { if(head != NULL) { Node* prevNode = head; Node* tempNode = head; Node* curNode = head->next; prevNode->next = prevNode; while(curNode != head) { tempNode = curNode->next; curNode->next = prevNode; head->next = curNode; prevNode = curNode; curNode = tempNode; } head = prevNode; } } //display the content of the list void PrintList() { Node* temp = head; if(temp != NULL) { cout<<"The list contains: "; while(true) { cout<<temp->data<<" "; temp = temp->next; if(temp == head) break; } cout<<endl; } else { cout<<"The list is empty.\n"; } } }; // test the code int main() { LinkedList MyList; //Add five elements in the list. MyList.push_back(10); MyList.push_back(20); MyList.push_back(30); MyList.push_back(40); MyList.push_back(50); //Display the content of the list. MyList.PrintList(); //Reversing the list. MyList.reverseList(); //Display the content of the list. MyList.PrintList(); return 0; }
The above code will give the following output:
The list contains: 10 20 30 40 50 The list contains: 50 40 30 20 10