Python Program to Count the Number of Occurrences of an Element in the Linked List using Recursion

When it is required to count the number of occurrence of a specific element of a linked list with the help of recursion, a method to add elements to the linked list, a method to print the elements of the linked list, and a method to count the occurrence of an element in the linked list is defined. A helper function is defined, since recursion is being used. This helper function calls the previously defined occurrence counting function.

Below is a demonstration for the same −

Example

 Live Demo

class Node:
   def __init__(self, data):
      self.data = data
      self.next = None

class my_linked_list:
   def __init__(self):
      self.head = None
      self.last_node = None

   def add_value(self, my_data):
      if self.last_node is None:
         self.head = Node(my_data)
         self.last_node = self.head
      else:
         self.last_node.next = Node(my_data)
      self.last_node = self.last_node.next

   def print_it(self):
      curr = self.head
      while curr:
         print(curr.data)
         curr = curr.next

   def count_val(self, key):
      return self.count_helper_fun(self.head, key)

   def count_helper_fun(self, curr, key):
      if curr is None:
         return 0

      if curr.data == key:
         return 1 + self.count_helper_fun(curr.next, key)
      else:
         return self.count_helper_fun(curr.next, key)

my_instance = my_linked_list()
my_list = [56, 43, 70, 67, 89, 91, 70, 23, 46, 70]
for elem in my_list:
   my_instance.add_value(elem)
print("The linked list contains the below elements:")
my_instance.print_it()

key_val = int(input('Enter the data item: '))
count_val = my_instance.count_val(key_val)
print('{0} occurs {1} time(s) in the list.'.format(key_val, count_val))

Output

The linked list contains the below elements:
56
43
70
67
89
91
70
23
46
70
Enter the data item: 70

70 occurs 3 time(s) in the list.

Explanation

• The ‘Node’ class is created.

• Another ‘my_linked_list’ class with required attributes is created.

• It has an ‘init’ function that is used to initialize the first element, i.e the ‘head’ to ‘None’ and last node to ‘None’.

• Another method named ‘add_value’ is defined, that is used to add data to the linked list.

• Another method named ‘print_it’ is defined, that iterates over the list, and prints the elements.

• Another method named ‘count_val’ is defined that is used to call the helper function.

• Another helper function, named ‘count_helper_fun’ is defined, that helps determine the frequency of occurrence of a specific element in the linked list.

• An object of the ‘my_linked_list’ class is created.

• The count_val method is called, to find the frequency of a specific element.

• This output is displayed on the console.