Delete nodes which have a greater value on right side using recursion

Given a singly linked list, remove all the nodes which have a greater value on the right side.

Examples:
a) The list 12->15->10->11->5->6->2->3->NULL should be changed to 15->11->6->3->NULL. Note that 12, 10, 5 and 2 have been deleted because there is a greater value on the right side.

When we examine 12, we see that after 12 there is one node with a value greater than 12 (i.e. 15), so we delete 12.
When we examine 15, we find no node after 15 that has a value greater than 15 so we keep this node.
When we go like this, we get 15->6->3



b) The list 10->20->30->40->50->60->NULL should be changed to 60->NULL. Note that 10, 20, 30, 40 and 50 have been deleted because they all have a greater value on the right side.

c) The list 60->50->40->30->20->10->NULL should not be changed.

Approach: We have already solved this problem by using 2 loops and reversing linked list in the post Delete nodes which have a greater value on right side

Here we will discuss the solution without reversing the list. We will use recursion to solve this problem in which the base case would be when the head is pointing to NULL. Else we would be recursively calling function for the next node and updating max value if currentNode->data > currentMax. In this way, the whole list would be updated.

Below is the implementation of the above approach:

C++

filter_none

edit
close

play_arrow

link
brightness_4
code

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
  
/* structure of a linked list node */
struct Node {
    int data;
    struct Node* next;
};
  
/*Utility function to find maximum value*/
int maxVal(int a, int b)
{
    if (a > b)
        return a;
    return b;
}
  
/* Function to delete nodes which have 
a node with greater value node 
on left side */
struct Node* delNodes(struct Node* head, int* max)
{
  
    // Base case
    if (head == NULL) {
        return head;
    }
  
    head->next = delNodes(head->next, max);
    if (head->data < *max) {
        return head->next;
    }
    *max = maxVal(head->data, *max);
    return head;
}
  
/* Utility function to insert a node at the beginning */
void push(struct Node** head, int new_data)
{
    struct Node* new_node = new Node;
    new_node->data = new_data;
    new_node->next = *head;
    *head = new_node;
}
  
/* Utility function to print a linked list */
void printList(struct Node* head)
{
    while (head != NULL) {
        cout << head->data << " ";
        head = head->next;
    }
    cout << endl;
}
  
/* Driver program to test above functions */
int main()
{
    struct Node* head = NULL;
  
    /* Create following linked list 
    12->15->10->11->5->6->2->3 */
    push(&head, 3);
    push(&head, 2);
    push(&head, 6);
    push(&head, 5);
    push(&head, 11);
    push(&head, 10);
    push(&head, 15);
    push(&head, 12);
  
    cout << "Given Linked List" << endl;
    printList(head);
    int max = INT_MIN;
    head = delNodes(head, &max);
  
    cout << "Modified Linked List" << endl;
    printList(head);
  
    return 0;
}

chevron_right


Java

filter_none

edit
close

play_arrow

link
brightness_4
code

// Java implementation of the approach 
class GFG 
{
  
/* structure of a linked list node */
static class Node
    int data; 
    Node next; 
}; 
static Node head;
static int max;
  
/*Utility function to find maximum value*/
static int maxVal(int a, int b) 
    if (a > b) 
        return a; 
    return b; 
  
/* Function to delete nodes which have 
a node with greater value node 
on left side */
static Node delNodes(Node head) 
  
    // Base case 
    if (head == null
    
        return head; 
    
  
    head.next = delNodes(head.next); 
    if (head.data < max)
    
        return head.next; 
    
    max = maxVal(head.data, max); 
    return head; 
  
/* Utility function to insert a node
at the beginning */
static void push(Node head_ref, 
                 int new_data) 
    Node new_node = new Node(); 
    new_node.data = new_data; 
    new_node.next = head_ref; 
    head_ref = new_node; 
        head = head_ref;
  
/* Utility function to print a linked list */
static void printList(Node head) 
    while (head != null
    
        System.out.print(head.data + " "); 
        head = head.next; 
    
    System.out.println(); 
  
// Driver Code
public static void main(String[] args) 
{
    head = null
  
    /* Create following linked list 
    12.15.10.11.5.6.2.3 */
    push(head, 3); 
    push(head, 2); 
    push(head, 6); 
    push(head, 5); 
    push(head, 11); 
    push(head, 10); 
    push(head, 15); 
    push(head, 12); 
  
    System.out.println("Given Linked List"); 
    printList(head); 
    max = Integer.MIN_VALUE; 
    head = delNodes(head); 
  
    System.out.println("Modified Linked List"); 
    printList(head);
}
  
// This code is contributed by 29AjayKumar

chevron_right


Python3

filter_none

edit
close

play_arrow

link
brightness_4
code

# Python3 program to reverse a linked
# list using a stack 
  
# Link list node 
class Node:
      
    def __init__(self, data, next):
        self.data = data
        self.next = next
  
class LinkedList:
      
    def __init__(self):
        self.head = None
          
    # Function to push a new Node in
    # the linked list 
    def push(self, new_data): 
      
        new_node = Node(new_data, self.head) 
        self.head = new_node
      
    # Function to delete nodes which have a node
    # with greater value node on left side
    def delNodes(self, head): 
      
        # Base case 
        if head == None
            return head
          
        global Max
      
        head.next = self.delNodes(head.next)
        if head.data < Max
            return head.next
          
        Max = max(head.data, Max
        return head 
      
    # Function to print the Linked list 
    def printList(self):
          
        curr = self.head
        while curr: 
            print(curr.data, end = " "
            curr = curr.next
        print()
  
# Driver Code 
if __name__ == "__main__":
  
    # Start with the empty list
    linkedList = LinkedList() 
  
    # Create following linked list 
    # 12->15->10->11->5->6->2->3 
    linkedList.push(3
    linkedList.push(2
    linkedList.push(6
    linkedList.push(5
    linkedList.push(11
    linkedList.push(10
    linkedList.push(15
    linkedList.push(12
  
    print("Given Linked List")
    linkedList.printList() 
    Max = float('-inf')
    linkedList.head = linkedList.delNodes(linkedList.head)
  
    print("Modified Linked List")
      
    linkedList.printList() 
  
# This code is contributed by Rituraj Jain 

chevron_right


C#

filter_none

edit
close

play_arrow

link
brightness_4
code

// C# implementation of the approach
using System;
  
class GFG 
{
  
/* structure of a linked list node */
public class Node
    public int data; 
    public Node next; 
}; 
static Node head;
static int max;
  
/*Utility function to find maximum value*/
static int maxVal(int a, int b) 
    if (a > b) 
        return a; 
    return b; 
  
/* Function to delete nodes which have 
a node with greater value node 
on left side */
static Node delNodes(Node head) 
  
    // Base case 
    if (head == null
    
        return head; 
    
  
    head.next = delNodes(head.next); 
    if (head.data < max)
    
        return head.next; 
    
    max = maxVal(head.data, max); 
    return head; 
  
/* Utility function to insert a node
at the beginning */
static void push(Node head_ref, 
                 int new_data) 
    Node new_node = new Node(); 
    new_node.data = new_data; 
    new_node.next = head_ref; 
    head_ref = new_node; 
        head = head_ref;
  
/* Utility function to print a linked list */
static void printList(Node head) 
    while (head != null
    
        Console.Write(head.data + " "); 
        head = head.next; 
    
    Console.WriteLine(); 
  
// Driver Code
public static void Main(String[] args) 
{
    head = null
  
    /* Create following linked list 
    12.15.10.11.5.6.2.3 */
    push(head, 3); 
    push(head, 2); 
    push(head, 6); 
    push(head, 5); 
    push(head, 11); 
    push(head, 10); 
    push(head, 15); 
    push(head, 12); 
  
    Console.WriteLine("Given Linked List"); 
    printList(head); 
    max = int.MinValue; 
    head = delNodes(head); 
  
    Console.WriteLine("Modified Linked List"); 
    printList(head);
}
}
  
// This code is contributed by PrinciRaj1992

chevron_right


Output:

Given Linked List
12 15 10 11 5 6 2 3 
Modified Linked List
15 11 6 3


My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.





Article Tags :
Practice Tags :


Be the First to upvote.


Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.