# C Program For Removing Duplicates From A Sorted Linked List

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``#include` `// Link list node``struct` `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 of 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 code``int` `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.
Auxiliary Space: O(1)

Recursive Approach :

## C

 `// C recursive Program to remove duplicates``// from a sorted linked list``#include``#include` `// 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 of 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 code``int` `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```

Time Complexity: O(n), where n is the number of nodes in the given linked list.
Auxiliary Space: O(n), due to recursive stack where n is the number of nodes in the given linked list.

Please refer complete article on Remove duplicates from a sorted linked list for more details!

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