Next Greater Element in a Circular Linked List

Given a circular singly linked list, the task is to print the next greater element for each node in the linked list. If there is no next greater element for any node, then print “-1” for that node.

Examples:

Input: head = 1 ? 5 ? 2 ? 10 ? 0 ? (head)
Output: 5 10 10 -1 -1
Explanation:
The next greater elements of each node are:

1. Node 1: Next greater element is 5.
2. Node 5: Next greater element is 10.
3. Node 2: Next greater element is 10.
4. Node 10: No next greater element exists. Therefore print “-1”.
5. Node 0: No next greater element exists. Therefore print “-1”.

Input: head = 1 ? 5 ? 12 ? 10 ? 0 ? (head)
Output: 5 12 -1 12 -1

Approach: The given problem can be solved by iterating the circular linked list to the right until the same element appears again and then printing the next greater element found for each node. Follow the steps below to solve the problem:

• Initialize a Node variable, say res as NULL to store the head node of the Linked List representing the next greater element.
• Also, initialize a Node variable, say tempList as NULL to iterate over the Linked List representing the next greater element.
• Iterate over the circular Linked List and perform the following steps:
  • Initialize a Node, say curr to store the reference to the current node of the circular linked list.
  • Initialize a variable say Val as -1 to store the value of the next greater element of the current node.
  • Perform the following steps until curr is not equal to the reference of the current node of the circular Linked List:
    • If the value at the current node curr is greater than the value of the current node of the circular Linked list then assign the value of curr.data to Val and break out of the loop.
    • Update the value of curr node as curr = curr.next.
  • If the node res is NULL then create a new node with the value Val and assign it to res, and then assign the value of res to tempList.
  • Otherwise, create a new node with the value Val and assign it to tempList.next, and then update the node tempList as tempList = tempList.next.
• After completing the above steps, print the elements of the Linked List res as the answer.

Below is the implementation of the above approach:

C++

// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;

// Node structure of the circular
// Linked list
struct Node
{
    int data;
    Node *next;

    // Constructor
    Node(int d)
    {
        data = d;
        next = NULL;
    }
};

// Function to print the elements
// of a Linked list
void print(Node *head)
{
    Node *temp = head;

    // Iterate the linked list
    while (temp != NULL)
    {

        // Print the data
        cout << temp->data << " ";
        temp = temp->next;
    }
}

// Function to find the next
// greater element for each node
void NextGreaterElement(Node *head)
{

    // Stores the head of circular
    // Linked list
    Node *H = head;

    // Stores the head of the
    // resulting linked list
    Node *res = NULL;

    // Stores the temporary head of
    // the resulting Linked list
    Node *tempList = NULL;

    // Iterate until head
    // is not equal to H
    do
    {

        // Used to iterate over the
        // circular Linked List
        Node *curr = head;

        // Stores if there exist
        // any next Greater element
        int Val = -1;

        // Iterate until head is
        // not equal to curr
        do
        {

            // If the current node
            // is smaller
            if (head->data < curr->data)
            {

                // Update the value
                // of Val
                Val = curr->data;
                break;
            }

            // Update curr
            curr = curr->next;

        } while (curr != head);

        // If res is Null
        if (res == NULL)
        {

            // Create new Node with
            // value curr->data
            res = new Node(Val);

            // Assign address of
            // res to tempList
            tempList = res;
        }
        else
        {

            // Update tempList
            tempList->next = new Node(Val);

            // Assign address of the
            // next node to tempList
            tempList = tempList->next;
        }

        // Update the value of
        // head node
        head = head->next;
    } while (head != H);

    // Print the resulting
    // Linked list
    print(res);
}

// Driver code
int main()
{
    Node *head = new Node(1);
    head->next = new Node(5);
    head->next->next = new Node(12);
    head->next->next->next = new Node(10);

    head->next->next->next->next = new Node(0);
    head->next->next->next->next->next = head;

    NextGreaterElement(head);

    return 0;
}

// This code is contributed by mohit kumar 29

Java

// Java program for the above approach

import java.io.*;
import java.util.*;

class GFG {
    static Node head;

    // Node structure of the circular
    // Linked list
    static class Node {
        int data;
        Node next;

        // Constructor
        public Node(int data)
        {
            this.data = data;
            this.next = null;
        }
    }

    // Function to print the elements
    // of a Linked list
    static void print(Node head)
    {
        Node temp = head;

        // Iterate the linked list
        while (temp != null) {

            // Print the data
            System.out.print(
                temp.data + " ");
            temp = temp.next;
        }
    }

    // Function to find the next
    // greater element for each node
    static void NextGreaterElement(Node head)
    {
        // Stores the head of circular
        // Linked list
        Node H = head;

        // Stores the head of the
        // resulting linked list
        Node res = null;

        // Stores the temporary head of
        // the resulting Linked list
        Node tempList = null;

        // Iterate until head
        // is not equal to H
        do {

            // Used to iterate over the
            // circular Linked List
            Node curr = head;

            // Stores if there exist
            // any next Greater element
            int Val = -1;

            // Iterate until head is
            // not equal to curr
            do {

                // If the current node
                // is smaller
                if (head.data < curr.data) {

                    // Update the value
                    // of Val
                    Val = curr.data;
                    break;
                }

                // Update curr
                curr = curr.next;

            } while (curr != head);

            // If res is Null
            if (res == null) {

                // Create new Node with
                // value curr.data
                res = new Node(Val);

                // Assign address of
                // res to tempList
                tempList = res;
            }
            else {

                // Update tempList
                tempList.next = new Node(Val);

                // Assign address of the
                // next node to tempList
                tempList = tempList.next;
            }

            // Update the value of
            // head node
            head = head.next;
        } while (head != H);

        // Print the resulting
        // Linked list
        print(res);
    }

    // Driver Code
    public static void main(String[] args)
    {
        head = new Node(1);
        head.next = new Node(5);
        head.next.next = new Node(12);
        head.next.next.next = new Node(10);

        head.next.next.next.next = new Node(0);
        head.next.next.next.next.next = head;

        NextGreaterElement(head);
    }
}

Python3

# Python program for the above approach

# Node structure of the circular
    # Linked list
class Node:

# Constructor
    def __init__(self, data):

        self.data = data
        self.next = None

# Function to print the elements
    # of a Linked list
def Print(head):
    temp = head

    # Iterate the linked list
    while (temp != None):

        # Print the data
        print(temp.data,end=" ")
        temp = temp.next

# Function to find the next
    # greater element for each node
def NextGreaterElement(head):

    # Stores the head of circular
    # Linked list
    H = head

    # Stores the head of the
    # resulting linked list
    res = None

    # Stores the temporary head of
    # the resulting Linked list
    tempList = None

    # Iterate until head
    # is not equal to H
    while(True):

        # Used to iterate over the
        # circular Linked List
        curr = head

        # Stores if there exist
        # any next Greater element
        Val = -1

        # Iterate until head is
        # not equal to curr

        # If the current node
        # is smaller
        while(True):
            if (head.data < curr.data):

                # Update the value
                # of Val
                Val = curr.data
                break

            # Update curr
            curr = curr.next

            if(curr == head):
                break

        # If res is Null
        if (res == None):

            # Create new Node with
            # value curr.data
            res = Node(Val)

            # Assign address of
            # res to tempList
            tempList = res

        else:

            # Update tempList
            tempList.next = Node(Val)

            # Assign address of the
            # next node to tempList
            tempList = tempList.next


        # Update the value of
        # head node
        head = head.next
        if(head == H):
            break

    # Print the resulting
    # Linked list
    Print(res)

# Driver Code
head = Node(1)
head.next = Node(5)
head.next.next = Node(12)
head.next.next.next = Node(10)

head.next.next.next.next = Node(0)
head.next.next.next.next.next = head

NextGreaterElement(head)

# This code is contributed by shinjanpatra

C#

// C# program for the above approach
using System;

// Node structure of the circular
// Linked list
class Node
{
    public int data;
    public Node next;

    // Constructor
    public Node(int data)
    {
        this.data = data;
        this.next = null;
    }
}

class GFG{

static Node head;

// Function to print the elements
// of a Linked list
static void print(Node head)
{
    Node temp = head;

    // Iterate the linked list
    while (temp != null)
    {

        // Print the data
        Console.Write(temp.data + " ");
        temp = temp.next;
    }
}

// Function to find the next
// greater element for each node
static void NextGreaterElement(Node head)
{

    // Stores the head of circular
    // Linked list
    Node H = head;

    // Stores the head of the
    // resulting linked list
    Node res = null;

    // Stores the temporary head of
    // the resulting Linked list
    Node tempList = null;

    // Iterate until head
    // is not equal to H
    do{

        // Used to iterate over the
        // circular Linked List
        Node curr = head;

        // Stores if there exist
        // any next Greater element
        int Val = -1;

        // Iterate until head is
        // not equal to curr
        do{

            // If the current node
            // is smaller
            if (head.data < curr.data)
            {

                // Update the value
                // of Val
                Val = curr.data;
                break;
            }

            // Update curr
            curr = curr.next;

        } while (curr != head);

        // If res is Null
        if (res == null)
        {

            // Create new Node with
            // value curr.data
            res = new Node(Val);

            // Assign address of
            // res to tempList
            tempList = res;
        }
        else
        {

            // Update tempList
            tempList.next = new Node(Val);

            // Assign address of the
            // next node to tempList
            tempList = tempList.next;
        }

        // Update the value of
        // head node
        head = head.next;
    } while (head != H);

    // Print the resulting
    // Linked list
    print(res);
}

// Driver Code
static public void Main()
{
    head = new Node(1);
    head.next = new Node(5);
    head.next.next = new Node(12);
    head.next.next.next = new Node(10);

    head.next.next.next.next = new Node(0);
    head.next.next.next.next.next = head;

    NextGreaterElement(head);
}
}

// This code is contributed by unknown2108

Javascript

<script>
// Javascript program for the above approach

let head;

// Node structure of the circular
    // Linked list
class Node
{
// Constructor
    constructor(data)
    {
        this.data = data;
        this.next = null;
    }
}

// Function to print the elements
    // of a Linked list
function print(head)
{
    let temp = head;

        // Iterate the linked list
        while (temp != null) {

            // Print the data
            document.write(
                temp.data + " ");
            temp = temp.next;
        }
}

// Function to find the next
    // greater element for each node
function NextGreaterElement(head)
{
    // Stores the head of circular
        // Linked list
        let H = head;

        // Stores the head of the
        // resulting linked list
        let res = null;

        // Stores the temporary head of
        // the resulting Linked list
        let tempList = null;

        // Iterate until head
        // is not equal to H
        do {

            // Used to iterate over the
            // circular Linked List
            let curr = head;

            // Stores if there exist
            // any next Greater element
            let Val = -1;

            // Iterate until head is
            // not equal to curr
            do {

                // If the current node
                // is smaller
                if (head.data < curr.data) {

                    // Update the value
                    // of Val
                    Val = curr.data;
                    break;
                }

                // Update curr
                curr = curr.next;

            } while (curr != head);

            // If res is Null
            if (res == null) {

                // Create new Node with
                // value curr.data
                res = new Node(Val);

                // Assign address of
                // res to tempList
                tempList = res;
            }
            else {

                // Update tempList
                tempList.next = new Node(Val);

                // Assign address of the
                // next node to tempList
                tempList = tempList.next;
            }

            // Update the value of
            // head node
            head = head.next;
        } while (head != H);

        // Print the resulting
        // Linked list
        print(res);
}

// Driver Code
head = new Node(1);
head.next = new Node(5);
head.next.next = new Node(12);
head.next.next.next = new Node(10);

head.next.next.next.next = new Node(0);
head.next.next.next.next.next = head;

NextGreaterElement(head);


// This code is contributed by unknown2108
</script>

Output

5 12 -1 12 1 

Time Complexity: O(N²)
Auxiliary Space: O(N)