Java - Count nodes in the Circular Singly Linked List

Counting nodes in a circular singly linked list is very useful while working on it. It requires creating a temp node pointing to the head of the list and a variable called i with initial value 0. If the temp node is not null, increase i by 1 and move to the next node using temp next. Repeat the process till the temp node reaches the head. The final value of i will be the total number of nodes in the circular singly linked list.

The function countNodes is created for this purpose. It is a 4-step process.

int countNodes() {
  
  //1. create a temp node pointing to head
  Node temp = new Node();
  temp = this.head;

  //2. create a variable to count nodes
  int i = 0;

  //3. if the temp node is not null increase 
  //   i by 1 and move to the next node, repeat
  //   the process till the temp becomes null
  if (temp != null) {
    i++;
    temp = temp.next;
  }
  while(temp != this.head) {
    i++;
    temp = temp.next;
  }  

  //4. return the count
  return i;  
}

The below is a complete program that uses above discussed concept of counting the total number of nodes of a circular singly linked list.

//node structure
class Node {
    int data;
    Node next;
};

class LinkedList {
  Node head;

  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 = new Node();
      temp = head;
      while(temp.next != head)
        temp = temp.next;
      temp.next = newNode;
      newNode.next = head;
    }    
  }

  //count nodes in the list
  int countNodes() {
    Node temp = new Node();
    temp = this.head;
    int i = 0;
    if (temp != null) {
      i++;
      temp = temp.next;
    }
    while(temp != this.head) {
      i++;
      temp = temp.next;
    }  
    return i;  
  }  

  //display the content of the list
  void PrintList() {
    Node temp = new Node();
    temp = this.head;
    if(temp != null) {
      System.out.print("The list contains: ");
      while(true) {
        System.out.print(temp.data + " ");
        temp = temp.next;
        if(temp == this.head)
          break;
      }
      System.out.println();
    } else {
      System.out.println("The list is empty.");
    }
  }     
};

// test the code 
public class Implementation {
  public static void main(String[] args) {
    LinkedList MyList = new LinkedList();

    //Add four elements in the list.
    MyList.push_back(10);
    MyList.push_back(20);
    MyList.push_back(30);
    MyList.push_back(40);

    //Display the content of the list.
    MyList.PrintList();

    //number of nodes in the list
    System.out.println("No. of nodes: "+ MyList.countNodes());
  }
}

The above code will give the following output:

The list contains: 10 20 30 40
No. of nodes: 4