Given a singly linked list, find middle of the linked list and set middle node of the linked list at beginning of the linked list.

Examples:

Input : 1 2 3 4 5 Output : 3 1 2 4 5 Input : 1 2 3 4 5 6 Output : 4 1 2 3 5 6

The idea is to first find middle of a linked list using two pointers, first one moves one at a time and second one moves two at a time. When second pointer reaches end, first reaches middle. We also keep track of previous of first pointer so that we can remove middle node from its current position and can make it head.

## C

// C program to make middle node as head of // linked list. #include <stdio.h> #include <stdlib.h> /* Link list node */ struct Node { int data; struct Node* next; }; /* Function to get the middle and set at beginning of the linked list*/ void setMiddleHead(struct Node** head) { if (*head == NULL) return; // To traverse list nodes one by one struct Node* one_node = (*head); // To traverse list nodes by skipping // one. struct Node* two_node = (*head); // To keep track of previous of middle struct Node* prev = NULL; while (two_node != NULL && two_node->next != NULL) { /* for previous node of middle node */ prev = one_node; /* move one node each time*/ two_node = two_node->next->next; /* move two node each time*/ one_node = one_node->next; } /* set middle node at head */ prev->next = prev->next->next; one_node->next = (*head); (*head) = one_node; } // To insert a node at the beginning of linked // list. void push(struct Node** head_ref, int new_data) { /* allocate node */ struct Node* new_node = (struct Node*)malloc(sizeof(struct Node)); 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; } // A function to print a given linked list void printList(struct Node* ptr) { while (ptr != NULL) { printf("%d ", ptr->data); ptr = ptr->next; } printf("\n"); } /* Driver function*/ int main() { // Create a list of 5 nodes struct Node* head = NULL; int i; for (i = 5; i > 0; i--) push(&head, i); printf(" list before: "); printList(head); setMiddleHead(&head); printf(" list After: "); printList(head); return 0; }

## Java

// Java program to make middle node // as head of Linked list public class GFG { /* Link list node */ static class Node { int data; Node next; Node(int data){ this.data = data; next = null; } } static Node head; /* Function to get the middle and set at beginning of the linked list*/ static void setMiddleHead() { if (head == null) return; // To traverse list nodes one // by one Node one_node = head; // To traverse list nodes by // skipping one. Node two_node = head; // To keep track of previous of middle Node prev = null; while (two_node != null && two_node.next != null) { /* for previous node of middle node */ prev = one_node; /* move one node each time*/ two_node = two_node.next.next; /* move two node each time*/ one_node = one_node.next; } /* set middle node at head */ prev.next = prev.next.next; one_node.next = head; head = one_node; } // To insert a node at the beginning of // linked list. static void push(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; } // A function to print a given linked list static void printList(Node ptr) { while (ptr != null) { System.out.print(ptr.data+" "); ptr = ptr.next; } System.out.println(); } /* Driver function*/ public static void main(String args[]) { // Create a list of 5 nodes head = null; int i; for (i = 5; i > 0; i--) push(i); System.out.print(" list before: "); printList(head); setMiddleHead(); System.out.print(" list After: "); printList(head); } } // This code is contributed by Sumit Ghosh

Output:

list before: 1 2 3 4 5 list After : 3 1 2 4 5

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