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First non-repeating in a linked list
  • Difficulty Level : Easy
  • Last Updated : 16 Dec, 2020

Given a linked list, find its first non-repeating integer element.

Examples: 

Input : 10->20->30->10->20->40->30->NULL
Output :First Non-repeating element is 40.

Input :1->1->2->2->3->4->3->4->5->NULL
Output :First Non-repeating element is 5.

Input :1->1->2->2->3->4->3->4->NULL
Output :No NOn-repeating element is found.

1) Create a hash table and marked all element as zero. 
2) Traverse the linked list and count the frequency of all the element in hashtable. 
3) Traverse the linked list again and see the element who’s frequency is 1 in hashtable. 

C++

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// C++ program to find first non-repeating 
// element in a linked list
#include<bits/stdc++.h>
using namespace std;
  
/* Link list node */
struct Node
{
    int data;
    struct Node* next;
};
  
/* Function to find the first non-repeating
 element in  the linked list */
int firstNonRepeating(struct Node *head)
{
    // Create an empty map and insert all linked
    // list elements into hash table
    unordered_map<int, int> mp;
    for (Node *temp=head; temp!=NULL; temp=temp->next)
       mp[temp->data]++;    
     
    // Traverse the linked list again and return
    // the first node whose count is 1
    for (Node *temp=head; temp!=NULL; temp=temp->next)
       if (mp[temp->data] == 1)
           return temp->data;
  
    return -1;
}
  
/* Function to push a node */
void push(struct Node** head_ref, int new_data)
{
    struct Node* new_node =
            (struct Node*) malloc(sizeof(struct Node));
    new_node->data  = new_data;
    new_node->next = (*head_ref);    
    (*head_ref)    = new_node;
}
  
/* Driver program to test above function*/
int main()
{
     // Let us create below linked list.
     // 85->15->18->20->85->35->4->20->NULL 
     struct Node* head = NULL;
     push(&head, 20);
     push(&head, 4);
     push(&head, 35); 
     push(&head, 85);  
     push(&head, 20);
     push(&head, 18);
     push(&head, 15); 
     push(&head, 85);     
     cout << firstNonRepeating(head);                          
     return 0;

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Python3

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# Python3 program to find first non-repeating 
# element in a linked list
  
# Link list node 
class Node:
      
    def __init__(self, data):
        self.data = data
        self.next = None
          
# Function to find the first non-repeating
# element in  the linked list 
def firstNonRepeating(head):
      
    # Create an empty map and insert all linked
    # list elements into hash table
    mp = dict()
    temp = head
      
    while (temp != None):
        if temp.data not in mp:
            mp[temp.data] = 0
              
        mp[temp.data] += 1
        temp = temp.next
      
    temp = head
      
    # Traverse the linked list again and return
    # the first node whose count is 1
    while (temp != None):
        if temp.data in mp:
            if mp[temp.data] == 1:
                return temp.data
                  
        temp = temp.next
   
    return -1
  
# Function to push a node 
def push(head_ref, new_data):
  
    new_node = Node(new_data)
    new_node.next = head_ref
    head_ref = new_node
      
    return head_ref
  
# Driver code
if __name__=='__main__':
      
    # Let us create below linked list.
    # 85->15->18->20->85->35->4->20->NULL 
    head = None
    head = push(head, 20)
    head = push(head, 4)
    head = push(head, 35
    head = push(head, 85)  
    head = push(head, 20)
    head = push(head, 18)
    head = push(head, 15)
    head = push(head, 85)
      
    print(firstNonRepeating(head))
       
# This code is contributed by rutvik_56

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Output: 

15

Further Optimizations: 
The above solution requires two traversals of linked list. In case we have many repeating elements, we can save one traversal by storing positions also in hash table. Please refer last method of Given a string, find its first non-repeating character for details. 

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