Given a singly linked list and a key, count number of occurrences of given key in linked list. For example, if given linked list is 1->2->1->2->1->3->1 and given key is 1, then output should be 4.
Method 1- Without Recursion
Algorithm:
1. Initialize count as zero.
2. Loop through each element of linked list:
a) If element data is equal to the passed number then
increment the count.
3. Return count.
Implementation:
C++
// C++ program to count occurrences in a linked list #include <bits/stdc++.h> using namespace std; /* Link list node */class Node { public: int data; Node* next; }; /* Given a reference (pointer to pointer) to the head of a list and an int, push a new node on the front of the list. */void push(Node** head_ref, int new_data) { /* allocate node */ Node* new_node = new Node(); /* put in the data */ new_node->data = new_data; /* link the old list off the new node */ new_node->next = (*head_ref); /* move the head to point to the new node */ (*head_ref) = new_node; } /* Counts the no. of occurences of a node (search_for) in a linked list (head)*/int count(Node* head, int search_for) { Node* current = head; int count = 0; while (current != NULL) { if (current->data == search_for) count++; current = current->next; } return count; } /* Drier program to test count function*/int main() { /* Start with the empty list */ Node* head = NULL; /* Use push() to construct below list 1->2->1->3->1 */ push(&head, 1); push(&head, 3); push(&head, 1); push(&head, 2); push(&head, 1); /* Check the count function */ cout<<"count of 1 is "<<count(head, 1); return 0; } //This is code is contributed by rathbhupendra |
C
// C program to count occurrences in a linked list #include<stdio.h> #include<stdlib.h> /* Link list node */struct Node { int data; struct Node* next; }; /* Given a reference (pointer to pointer) to the head of a list and an int, push a new node on the front of the list. */void push(struct Node** head_ref, int new_data) { /* allocate node */ struct Node* new_node = (struct Node*) malloc(sizeof(struct Node)); /* put in the data */ new_node->data = new_data; /* link the old list off the new node */ new_node->next = (*head_ref); /* move the head to point to the new node */ (*head_ref) = new_node; } /* Counts the no. of occurences of a node (search_for) in a linked list (head)*/int count(struct Node* head, int search_for) { struct Node* current = head; int count = 0; while (current != NULL) { if (current->data == search_for) count++; current = current->next; } return count; } /* Drier program to test count function*/int main() { /* Start with the empty list */ struct Node* head = NULL; /* Use push() to construct below list 1->2->1->3->1 */ push(&head, 1); push(&head, 3); push(&head, 1); push(&head, 2); push(&head, 1); /* Check the count function */ printf("count of 1 is %d", count(head, 1)); return 0; } |
Java
// Java program to count occurrences in a linked list class LinkedList { Node head; // head of list /* Linked list Node*/ class Node { int data; Node next; Node(int d) {data = d; next = null; } } /* Inserts a new Node at front of the list. */ public void push(int new_data) { /* 1 & 2: Allocate the Node & Put in the data*/ Node new_node = new Node(new_data); /* 3. Make next of new Node as head */ new_node.next = head; /* 4. Move the head to point to new Node */ head = new_node; } /* Counts the no. of occurences of a node (search_for) in a linked list (head)*/ int count(int search_for) { Node current = head; int count = 0; while (current != null) { if (current.data == search_for) count++; current = current.next; } return count; } /* Drier function to test the above methods */ public static void main(String args[]) { LinkedList llist = new LinkedList(); /* Use push() to construct below list 1->2->1->3->1 */ llist.push(1); llist.push(2); llist.push(1); llist.push(3); llist.push(1); /*Checking count function*/ System.out.println("Count of 1 is "+llist.count(1)); } } // This code is contributed by Rajat Mishra |
Python
# Python program to count the number of time a given # int occurs in a linked list # Node class class Node: # Constructor to initialize the node object def __init__(self, data): self.data = data self.next = None class LinkedList: # Function to initialize head def __init__(self): self.head = None # Counts the no . of occurrences of a node # (search_for) in a linked list (head) def count(self, search_for): current = self.head count = 0 while(current is not None): if current.data == search_for: count += 1 current = current.next return count # Function to insert a new node at the beginning def push(self, new_data): new_node = Node(new_data) new_node.next = self.head self.head = new_node # Utility function to print the linked LinkedList def printList(self): temp = self.head while(temp): print temp.data, temp = temp.next # Driver program llist = LinkedList() llist.push(1) llist.push(3) llist.push(1) llist.push(2) llist.push(1) # Check for the count function print "count of 1 is %d" %(llist.count(1)) # This code is contributed by Nikhil Kumar Singh(nickzuck_007) |
C#
// C# program to count occurrences in a linked list using System; class LinkedList { Node head; // head of list /* Linked list Node*/ public class Node { public int data; public Node next; public Node(int d) {data = d; next = null; } } /* Inserts a new Node at front of the list. */ public void push(int new_data) { /* 1 & 2: Allocate the Node & Put in the data*/ Node new_node = new Node(new_data); /* 3. Make next of new Node as head */ new_node.next = head; /* 4. Move the head to point to new Node */ head = new_node; } /* Counts the no. of occurrences of a node (search_for) in a linked list (head)*/ int count(int search_for) { Node current = head; int count = 0; while (current != null) { if (current.data == search_for) count++; current = current.next; } return count; } /* Drier function to test the above methods */ public static void Main(String []args) { LinkedList llist = new LinkedList(); /* Use push() to construct below list 1->2->1->3->1 */ llist.push(1); llist.push(2); llist.push(1); llist.push(3); llist.push(1); /*Checking count function*/ Console.WriteLine("Count of 1 is "+llist.count(1)); } } // This code is contributed by Arnab Kundu |
Output:
count of 1 is 3
Time Complexity: O(n)
Auxiliary Space: O(1)
Method 2- With Recursion
This method is contributed by MY_DOOM.
Algorithm:
Algorithm count(head,key); if head is NULL return frequency if(head->data==key) increase frequency by 1 count(head->next,key)
Implementation:
C++
// C++ program to count occurrences in // a linked list using recursion #include <bits/stdc++.h> using namespace std; /* Link list node */struct Node { int data; struct Node* next; }; // global variable for counting frequeancy of // given element k int frequency = 0; /* Given a reference (pointer to pointer) to the head of a list and an int, push a new node on the front of the list. */void push(struct Node** head_ref, int new_data) { /* allocate node */ struct Node* new_node = (struct Node*)malloc(sizeof(struct Node)); /* put in the data */ new_node->data = new_data; /* link the old list off the new node */ new_node->next = (*head_ref); /* move the head to point to the new node */ (*head_ref) = new_node; } /* Counts the no. of occurences of a node (search_for) in a linked list (head)*/int count(struct Node* head, int key) { if (head == NULL) return frequency; if (head->data == key) frequency++; return count(head->next, key); } /* Driver program to test count function*/int main() { /* Start with the empty list */ struct Node* head = NULL; /* Use push() to construct below list 1->2->1->3->1 */ push(&head, 1); push(&head, 3); push(&head, 1); push(&head, 2); push(&head, 1); /* Check the count function */ cout<<"count of 1 is " <<count(head, 1); return 0; } |
Java
// Java program to count occurrences in // a linked list using recursion import java.io.*; import java.util.*; // Represents node of a linkedlist class Node { int data; Node next; Node(int val) { data = val; next = null; } } class GFG { // global variable for counting frequeancy of // given element k static int frequency = 0; /* Given a reference (pointer to pointer) to the head of a list and an int, push a new node on the front of the list. */ static Node push(Node head, int new_data) { // allocate node Node new_node = new Node(new_data); // link the old list off the new node new_node.next = head; // move the head to point to the new node head = new_node; return head; } /* Counts the no. of occurences of a node (search_for) in a linked list (head)*/ static int count(Node head, int key) { if (head == null) return frequency; if (head.data == key) frequency++; return count(head.next, key); } // Driver Code public static void main(String args[]) { // Start with the empty list Node head = null; /* Use push() to construct below list 1->2->1->3->1 */ head = push(head, 1); head = push(head, 3); head = push(head, 1); head = push(head, 2); head = push(head, 1); /* Check the count function */ System.out.print("count of 1 is " + count(head, 1)); } } // This code is contributed by rachana soma |
Python3
# Python program to count the number of # time a given int occurs in a linked list # Node class class Node: # Constructor to initialize the node object def __init__(self, data): self.data = data self.next = None class LinkedList: # Function to initialize head def __init__(self): self.head = None self.counter = 0 # Counts the no . of occurances of a node # (seach_for) in a linkded list (head) def count(self, li, key): # Base case if(not li): return self.counter # If key is present in # current node, return true if(li.data == key): self.counter = self.counter + 1 # Recur for remaining list return self.count(li.next, key) # Function to insert a new node # at the beginning def push(self, new_data): new_node = Node(new_data) new_node.next = self.head self.head = new_node # Utility function to print the # linked LinkedList def printList(self): temp = self.head while(temp): print (temp.data) temp = temp.next # Driver Code llist = LinkedList() llist.push(1) llist.push(3) llist.push(1) llist.push(2) llist.push(1) # Check for the count function print ("count of 1 is", llist.count(llist.head, 1)) # This code is contributed by # Gaurav Kumar Raghav |
Output:
count of 1 is 3
Below method can be used to avoid Global variable ‘frequency'(counter in case of Python 3 Code).
C++
// method can be used to avoid // Global variable 'frequency' /* Counts the no. of occurences of a node (search_for) in a linked list (head)*/int count(struct Node* head, int key) { if (head == NULL) return 0; if (head->data == key) return 1 + count(head->next, key); return count(head->next, key); } |
Java
// method can be used to avoid // Global variable 'frequency' /* Counts the no. of occurences of a node (search_for) in a linked list (head)*/int count(Node head, int key) { if (head == null) return 0; if (head.data == key) return 1 + count(head.next, key); return count(head.next, key); } // This code is contributed by rachana soma |
Python3
def count(self,temp, key): # during the initial call, temp # has the value of head # Base case if temp is None: return 0 # if a match is found, we # increment the counter if temp.data == key: return 1 + count(temp.next, key) return count(temp.next, key) # to call count, use # linked_list_name.count(head, key) |
The above method implements head recursion. Below given is the tail recursive implementation for the same. Thanks to Puneet Jain for suggesting this approach :
int count(struct Node* head, int key)
{
if(head == NULL)
return 0;
int frequency = count(head->next, key);
if(head->data == key)
return 1 + frequency;
// else
return frequency;
}
Time Complexity: O(n)
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