# Split a Circular Linked List into two halves `                         Original Linked List  ` `                         Result Linked List 1  ` `                         Result Linked List 2  `

If there are odd number of nodes, then first list should contain one extra.

Thanks to Geek4u for suggesting the algorithm.
1) Store the mid and last pointers of the circular linked list using tortoise and hare algorithm.
2) Make the second half circular.
3) Make the first half circular.
4) Set head (or start) pointers of the two linked lists.

In the below implementation, if there are odd nodes in the given circular linked list then the first result list has 1 more node than the second result list.

## C

 `/* Program to split a circular linked list into two halves */` `#include  ` `#include  ` ` `  `/* structure for a node */` `struct` `Node ` `{ ` `  ``int` `data; ` `  ``struct` `Node *next; ` `};  ` ` `  `/* Function to split a list (starting with head) into two lists. ` `   ``head1_ref and head2_ref are references to head nodes of  ` `    ``the two resultant linked lists */` `void` `splitList(``struct` `Node *head, ``struct` `Node **head1_ref,  ` `                                            ``struct` `Node **head2_ref) ` `{ ` `  ``struct` `Node *slow_ptr = head; ` `  ``struct` `Node *fast_ptr = head;  ` ` `  `  ``if``(head == NULL) ` `    ``return``; ` `  `  `  ``/* If there are odd nodes in the circular list then ` `     ``fast_ptr->next becomes head and for even nodes  ` `     ``fast_ptr->next->next becomes head */` `  ``while``(fast_ptr->next != head && ` `         ``fast_ptr->next->next != head)  ` `  ``{ ` `     ``fast_ptr = fast_ptr->next->next; ` `     ``slow_ptr = slow_ptr->next; ` `  ``}   ` ` `  ` ``/* If there are even elements in list then move fast_ptr */` `  ``if``(fast_ptr->next->next == head) ` `    ``fast_ptr = fast_ptr->next;       ` `   `  `  ``/* Set the head pointer of first half */` `  ``*head1_ref = head;     ` `      `  `  ``/* Set the head pointer of second half */` `  ``if``(head->next != head) ` `    ``*head2_ref = slow_ptr->next; ` `   `  `  ``/* Make second half circular */`    `  ``fast_ptr->next = slow_ptr->next; ` `   `  `  ``/* Make first half circular */`    `  ``slow_ptr->next = head;        ` `} ` ` `  `/* UTILITY FUNCTIONS */` `/* Function to insert a node at the begining of a Circular  ` `   ``linked lsit */` `void` `push(``struct` `Node **head_ref, ``int` `data) ` `{ ` `  ``struct` `Node *ptr1 = (``struct` `Node *)``malloc``(``sizeof``(``struct` `Node)); ` `  ``struct` `Node *temp = *head_ref;  ` `  ``ptr1->data = data;   ` `  ``ptr1->next = *head_ref;  ` `   `  `  ``/* If linked list is not NULL then set the next of  ` `    ``last node */` `  ``if``(*head_ref != NULL) ` `  ``{ ` `    ``while``(temp->next != *head_ref) ` `      ``temp = temp->next;         ` `    ``temp->next = ptr1;  ` `  ``} ` `  ``else` `     ``ptr1->next = ptr1; ``/*For the first node */` ` `  `  ``*head_ref = ptr1;      ` `}  ` ` `  `/* Function to print nodes in a given Circular linked list */` `void` `printList(``struct` `Node *head) ` `{ ` `  ``struct` `Node *temp = head;  ` `  ``if``(head != NULL) ` `  ``{ ` `    ``printf``(````" "````); ` `    ``do` `{ ` `      ``printf``(``"%d "``, temp->data); ` `      ``temp = temp->next; ` `    ``} ``while``(temp != head); ` `  ``} ` `} ` ` `  `/* Driver program to test above functions */` `int` `main() ` `{ ` `  ``int` `list_size, i;  ` `   `  `  ``/* Initialize lists as empty */` `  ``struct` `Node *head = NULL; ` `  ``struct` `Node *head1 = NULL; ` `  ``struct` `Node *head2 = NULL;   ` ` `  `  ``/* Created linked list will be 12->56->2->11 */` `  ``push(&head, 12);  ` `  ``push(&head, 56);    ` `  ``push(&head, 2);    ` `  ``push(&head, 11);    ` ` `  `  ``printf``(``"Original Circular Linked List"``); ` `  ``printList(head);       ` `  `  `  ``/* Split the list */`  `  ``splitList(head, &head1, &head2); ` `  `  `  ``printf``(````" First Circular Linked List"````); ` `  ``printList(head1);   ` ` `  `  ``printf``(````" Second Circular Linked List"````); ` `  ``printList(head2);   ` `   `  `  ``getchar``(); ` `  ``return` `0; ` `}  `

## Java

 `// Java program to delete a node from doubly linked list ` ` `  `class` `LinkedList { ` ` `  `    ``static` `Node head, head1, head2; ` ` `  `    ``static` `class` `Node { ` ` `  `        ``int` `data; ` `        ``Node next, prev; ` ` `  `        ``Node(``int` `d) { ` `            ``data = d; ` `            ``next = prev = ``null``; ` `        ``} ` `    ``} ` ` `  `    ``/* Function to split a list (starting with head) into two lists. ` `     ``head1_ref and head2_ref are references to head nodes of  ` `     ``the two resultant linked lists */` `    ``void` `splitList() { ` `        ``Node slow_ptr = head; ` `        ``Node fast_ptr = head; ` ` `  `        ``if` `(head == ``null``) { ` `            ``return``; ` `        ``} ` ` `  `        ``/* If there are odd nodes in the circular list then ` `         ``fast_ptr->next becomes head and for even nodes  ` `         ``fast_ptr->next->next becomes head */` `        ``while` `(fast_ptr.next != head ` `                ``&& fast_ptr.next.next != head) { ` `            ``fast_ptr = fast_ptr.next.next; ` `            ``slow_ptr = slow_ptr.next; ` `        ``} ` ` `  `        ``/* If there are even elements in list then move fast_ptr */` `        ``if` `(fast_ptr.next.next == head) { ` `            ``fast_ptr = fast_ptr.next; ` `        ``} ` ` `  `        ``/* Set the head pointer of first half */` `        ``head1 = head; ` ` `  `        ``/* Set the head pointer of second half */` `        ``if` `(head.next != head) { ` `            ``head2 = slow_ptr.next; ` `        ``} ` `        ``/* Make second half circular */` `        ``fast_ptr.next = slow_ptr.next; ` ` `  `        ``/* Make first half circular */` `        ``slow_ptr.next = head; ` `    ``} ` ` `  `    ``/* Function to print nodes in a given singly linked list */` `    ``void` `printList(Node node) { ` `        ``Node temp = node; ` `        ``if` `(node != ``null``) { ` `            ``do` `{ ` `                ``System.out.print(temp.data + ``" "``); ` `                ``temp = temp.next; ` `            ``} ``while` `(temp != node); ` `        ``} ` `    ``} ` ` `  `    ``public` `static` `void` `main(String[] args) { ` `        ``LinkedList list = ``new` `LinkedList(); ` ` `  `        ``//Created linked list will be 12->56->2->11 ` `        ``list.head = ``new` `Node(``12``); ` `        ``list.head.next = ``new` `Node(``56``); ` `        ``list.head.next.next = ``new` `Node(``2``); ` `        ``list.head.next.next.next = ``new` `Node(``11``); ` `        ``list.head.next.next.next.next = list.head; ` ` `  `        ``System.out.println(``"Original Circular Linked list "``); ` `        ``list.printList(head); ` ` `  `        ``// Split the list ` `        ``list.splitList(); ` `        ``System.out.println(``""``); ` `        ``System.out.println(``"First Circular List "``); ` `        ``list.printList(head1); ` `        ``System.out.println(``""``); ` `        ``System.out.println(``"Second Circular List "``); ` `        ``list.printList(head2); ` `         `  `    ``} ` `} ` ` `  `// This code has been contributed by Mayank Jaiswal `

## Python

 `# Python program to split circulart linked list into two halves ` ` `  `# A node structure ` `class` `Node: ` `     `  `    ``# Constructor to create a new node ` `    ``def` `__init__(``self``, data): ` `        ``self``.data ``=` `data ` `        ``self``.``next` `=` `None` ` `  ` `  `# Class to create a new  Circular Linked list ` `class` `CircularLinkedList: ` `     `  `    ``# Constructor to create a empty circular linked list ` `    ``def` `__init__(``self``): ` `        ``self``.head ``=` `None` ` `  `    ``# Function to insert a node at the beginning of a ` `    ``# circular linked list ` `    ``def` `push(``self``, data): ` `        ``ptr1 ``=` `Node(data) ` `        ``temp ``=` `self``.head ` `         `  `        ``ptr1.``next` `=` `self``.head ` ` `  `        ``# If linked list is not None then set the next of ` `        ``# last node ` `        ``if` `self``.head ``is` `not` `None``: ` `            ``while``(temp.``next` `!``=` `self``.head): ` `                ``temp ``=` `temp.``next`  `            ``temp.``next` `=` `ptr1 ` ` `  `        ``else``: ` `            ``ptr1.``next` `=` `ptr1 ``# For the first node ` ` `  `        ``self``.head ``=` `ptr1  ` ` `  `    ``# Function to print nodes in a given circular linked list ` `    ``def` `printList(``self``): ` `        ``temp ``=` `self``.head ` `        ``if` `self``.head ``is` `not` `None``: ` `            ``while``(``True``): ` `                ``print` `"%d"` `%``(temp.data), ` `                ``temp ``=` `temp.``next` `                ``if` `(temp ``=``=` `self``.head): ` `                    ``break`  ` `  ` `  `    ``# Function to split a list (starting with head) into  ` `    ``# two lists. head1 and head2 are the head nodes of the ` `    ``# two resultant linked lists ` `    ``def` `splitList(``self``, head1, head2): ` `        ``slow_ptr ``=` `self``.head  ` `        ``fast_ptr ``=` `self``.head ` `     `  `        ``if` `self``.head ``is` `None``: ` `            ``return`  `         `  `        ``# If htere are odd nodes in the circular list then ` `        ``# fast_ptr->next becomes head and for even nodes ` `        ``# fast_ptr->next->next becomes head ` `        ``while``(fast_ptr.``next` `!``=` `self``.head ``and`  `            ``fast_ptr.``next``.``next` `!``=` `self``.head ): ` `            ``fast_ptr ``=` `fast_ptr.``next``.``next` `            ``slow_ptr ``=` `slow_ptr.``next` ` `  `        ``# If there are event elements in list then ` `        ``# move fast_ptr ` `        ``if` `fast_ptr.``next``.``next` `=``=` `self``.head: ` `            ``fast_ptr ``=` `fast_ptr.``next` ` `  `        ``# Set the head pointer of first half ` `        ``head1.head ``=` `self``.head ` ` `  `        ``# Set the head pointer of second half ` `        ``if` `self``.head.``next` `!``=` `self``.head: ` `            ``head2.head ``=` `slow_ptr.``next` ` `  `        ``# Make second half circular ` `        ``fast_ptr.``next` `=` `slow_ptr.``next` `     `  `        ``# Make first half circular ` `        ``slow_ptr.``next` `=` `self``.head ` ` `  ` `  `# Driver program to test above functions ` ` `  `# Initialize lists as empty ` `head ``=` `CircularLinkedList()  ` `head1 ``=` `CircularLinkedList() ` `head2 ``=` `CircularLinkedList() ` ` `  `head.push(``12``) ` `head.push(``56``) ` `head.push(``2``) ` `head.push(``11``) ` ` `  `print` `"Original Circular Linked List"` `head.printList() ` ` `  `# Split the list  ` `head.splitList(head1 , head2) ` ` `  `print` ```" First Circular Linked List"``` `head1.printList() ` ` `  `print` ```" Second Circular Linked List"``` `head2.printList() ` ` `  `# This code is contributed by Nikhil Kumar Singh(nickzuck_007) `

Output:

```Original Circular Linked List
11 2 56 12
First Circular Linked List
11 2
Second Circular Linked List
56 12
```

Time Complexity: O(n)

Please write comments if you find any bug in above code/algorithm, or find other ways to solve the same problem

This article is attributed to GeeksforGeeks.org

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