C Program For Removing Duplicates From A Sorted Linked List - GeeksforGeeks

13 Dec, 2021
Write a function that takes a list sorted in non-decreasing order and deletes any duplicate nodes from the list. The list should only be traversed once. For example if the linked list is 11->11->11->21->43->43->60 then removeDuplicates() should convert the list to 11->21->43->60. 
Algorithm: Traverse the list from the head (or start) node. While traversing, compare each node with its next node. If the data of the next node is the same as the current node then delete the next node. Before we delete a node, we need to store the next pointer of the node 
Implementation: Functions other than removeDuplicates() are just to create a linked list and test removeDuplicates(). 
C
// C Program to remove duplicates // from a sorted linked list #include<stdio.h>#include<stdlib.h>  // Link list nodestruct Node{    int data;    struct Node* next;};  // The function removes duplicates // from a sorted list void removeDuplicates(struct Node* head){    // Pointer to traverse the linked list     struct Node* current = head;      // Pointer to store the next pointer     // of a node to be deleted    struct Node* next_next;         // Do nothing if the list is empty     if (current == NULL)        return;       // Traverse the list till last node     while (current->next != NULL)     {       // Compare current node with next node        if (current->data == current->next->data)        {           // The sequence of steps is important           next_next = current->next->next;           free(current->next);           current->next = next_next;         }         // This is tricky: only advance        // if no deletion       else        {          current = current->next;        }    }}  // UTILITY FUNCTIONS // Function to insert a node at the// beginning of the linked 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;}  // Function to print nodes in a given // linked list void printList(struct Node *node){    while (node!=NULL)    {       printf("%d ", node->data);       node = node->next;    }}   // Driver codeint main(){    // Start with the empty list     struct Node* head = NULL;        /* Let us create a sorted linked list        to test the functions. Created        linked list will be        11->11->11->13->13->20 */    push(&head, 20);    push(&head, 13);    push(&head, 13);      push(&head, 11);    push(&head, 11);    push(&head, 11);                                          printf(    "Linked list before duplicate removal  ");    printList(head);       // Remove duplicates from linked list     removeDuplicates(head);       printf(    "Linked list after duplicate removal ");             printList(head);                    return 0;}
Output:
Linked list before duplicate removal  11 11 11 13 13 20
Linked list after duplicate removal  11 13 20
Time Complexity: O(n) where n is the number of nodes in the given linked list.
Recursive Approach :  
C
// C recursive Program to remove duplicates// from a sorted linked list#include<stdio.h>#include<stdlib.h>  // Link list node struct Node{    int data;    struct Node* next;};  // UTILITY FUNCTIONS // Function to insert a node at // the beginning of the linked 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;}  // Function to print nodes in a // given linked list void printList(struct Node *node){    while (node!=NULL)    {       printf("%d ", node->data);       node = node->next;    }}   Node* deleteDuplicates(Node* head) {    if (head == nullptr)         return nullptr;      if (head->next == nullptr)         return head;      if (head->data == head->next->data)     {        Node *tmp;          // If find next element duplicate,         // preserve the next pointer to be         // deleted, skip it, and then delete         // the stored one. Return head        tmp = head->next;        head->next = head->next->next;        free(tmp);        return deleteDuplicates(head);    }      else     {        // if doesn't find next element duplicate, leave head         // and check from next element        head->next = deleteDuplicates(head->next);        return head;    }}  // Driver codeint main(){    // Start with the empty list     struct Node* head = NULL;        /* Let us create a sorted linked list        to test the functions. Created        linked list will be 11->11->11->13->13->20 */    push(&head, 20);    push(&head, 13);    push(&head, 13);      push(&head, 11);    push(&head, 11);    push(&head, 11);                                          printf(    "Linked list before duplicate removal  ");    printList(head);       // Remove duplicates from linked list     head = deleteDuplicates(head);       printf(    "Linked list after duplicate removal ");             printList(head);                    return 0;}// This code is contributed by Yogesh shukla 
Output:
Linked list before duplicate removal  11 11 11 13 13 20
Linked list after duplicate removal  11 13 20
Please refer complete article on Remove duplicates from a sorted linked list for more details!
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