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++

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// 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 begining */
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;
}

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Python3

# 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

Output:

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


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Improved By : rituraj_jain



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