Find the middle of a given linked list in C and Java

Given a singly linked list, find middle of the linked list. For example, if given linked list is 1->2->3->4->5 then output should be 3.
If there are even nodes, then there would be two middle nodes, we need to print second middle element. For example, if given linked list is 1->2->3->4->5->6 then output should be 4.

Method 1:
Traverse the whole linked list and count the no. of nodes. Now traverse the list again till count/2 and return the node at count/2.

Method 2:
Traverse linked list using two pointers. Move one pointer by one and other pointer by two. When the fast pointer reaches end slow pointer will reach middle of the linked list.

Below image shows how printMiddle function works in the code :

C

#include<stdio.h>  
#include<stdlib.h>  
  
/* Link list node */
struct Node  
{  
    int data;  
    struct Node* next;  
};  
  
/* Function to get the middle of the linked list*/
void printMiddle(struct Node *head)  
{  
    struct Node *slow_ptr = head;  
    struct Node *fast_ptr = head;  
  
    if (head!=NULL)  
    {  
        while (fast_ptr != NULL && fast_ptr->next != NULL)  
        {  
            fast_ptr = fast_ptr->next->next;  
            slow_ptr = slow_ptr->next;  
        }  
        printf("The middle element is [%d]\n\n", slow_ptr->data);  
    }  
}  
  
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;  
}  
  
// A utility function to print a given linked list  
void printList(struct Node *ptr)  
{  
    while (ptr != NULL)  
    {  
        printf("%d->", ptr->data);  
        ptr = ptr->next;  
    }  
    printf("NULL\n");  
}  
  
/* Driver program to test above function*/
int main()  
{  
    /* Start with the empty list */
    struct Node* head = NULL;  
    int i;  
  
    for (i=5; i>0; i--)  
    {  
        push(&head, i);  
        printList(head);  
        printMiddle(head);  
    }  
  
    return 0;  
}  

C++

#include<bits/stdc++.h>  
using namespace std;  
  
// Struct  
struct Node  
{  
    int data;  
    struct Node* next;  
};  
  
/* Function to get the middle of the linked list*/
void printMiddle(struct Node *head)  
{  
    struct Node *slow_ptr = head;  
    struct Node *fast_ptr = head;  
  
    if (head!=NULL)  
    {  
        while (fast_ptr != NULL && fast_ptr->next != NULL)  
        {  
            fast_ptr = fast_ptr->next->next;  
            slow_ptr = slow_ptr->next;  
        }  
        printf("The middle element is [%d]\n\n", slow_ptr->data);  
    }  
}  
  
// Function to add a new node  
void push(struct Node** head_ref, int new_data)  
{  
    /* allocate node */
    struct Node* new_node = new 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;  
}  
  
// A utility function to print a given linked list  
void printList(struct Node *ptr)  
{  
    while (ptr != NULL)  
    {  
        printf("%d->", ptr->data);  
        ptr = ptr->next;  
    }  
    printf("NULL\n");  
}  
  
// Driver Code 
int main()  
{  
    // Start with the empty list  
    struct Node* head = NULL;  
      
    // Iterate and add element  
    for (int i=5; i>0; i--)  
    {  
        push(&head, i);  
        printList(head);  
        printMiddle(head);  
    }  
  
    return 0;  
}  

Java

// Java program to find middle of linked list 
class LinkedList 
{ 
    Node head; // head of linked list 
  
    /* Linked list node */
    class Node 
    { 
        int data; 
        Node next; 
        Node(int d) 
        { 
            data = d; 
            next = null; 
        } 
    } 
  
    /* Function to print middle of linked list */
    void printMiddle() 
    { 
        Node slow_ptr = head; 
        Node fast_ptr = head; 
        if (head != null) 
        { 
            while (fast_ptr != null && fast_ptr.next != null) 
            { 
                fast_ptr = fast_ptr.next.next; 
                slow_ptr = slow_ptr.next; 
            } 
            System.out.println("The middle element is [" + 
                                slow_ptr.data + "] \n"); 
        } 
    } 
  
    /* Inserts a new Node at front of the list. */
    public void push(int new_data) 
    { 
        /* 1 & 2: Allocate the Node & 
                  Put in the data*/
        Node new_node = new Node(new_data); 
  
        /* 3. Make next of new Node as head */
        new_node.next = head; 
  
        /* 4. Move the head to point to new Node */
        head = new_node; 
    } 
  
    /* This function prints contents of linked list 
       starting from  the given node */
    public void printList() 
    { 
        Node tnode = head; 
        while (tnode != null) 
        { 
            System.out.print(tnode.data+"->"); 
            tnode = tnode.next; 
        } 
        System.out.println("NULL"); 
    } 
  
    public static void main(String [] args) 
    { 
        LinkedList llist = new LinkedList(); 
        for (int i=5; i>0; --i) 
        { 
            llist.push(i); 
            llist.printList(); 
            llist.printMiddle(); 
        } 
    } 
} 
// This code is contributed by Rajat Mishra 

Output:

5->NULL
The middle element is [5]

4->5->NULL
The middle element is [5]

3->4->5->NULL
The middle element is [4]

2->3->4->5->NULL
The middle element is [4]

1->2->3->4->5->NULL
The middle element is [3]

Method 3:
Initialize mid element as head and initialize a counter as 0. Traverse the list from head, while traversing increment the counter and change mid to mid->next whenever the counter is odd. So the mid will move only half of the total length of the list.
Thanks to Narendra Kangralkar for suggesting this method.

C

#include <stdio.h> 
#include <stdlib.h> 
  
/* Link list node */
struct node { 
    int data; 
    struct node* next; 
}; 
  
/* Function to get the middle of the linked list*/
void printMiddle(struct node* head) 
{ 
    int count = 0; 
    struct node* mid = head; 
  
    while (head != NULL) { 
        /* update mid, when 'count' is odd number */
        if (count & 1) 
            mid = mid->next; 
  
        ++count; 
        head = head->next; 
    } 
  
    /* if empty list is provided */
    if (mid != NULL) 
        printf("The middle element is [%d]\n\n", mid->data); 
} 
  
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; 
} 
  
// A utility function to print a given linked list 
void printList(struct node* ptr) 
{ 
    while (ptr != NULL) { 
        printf("%d->", ptr->data); 
        ptr = ptr->next; 
    } 
    printf("NULL\n"); 
} 
  
/* Driver program to test above function*/
int main() 
{ 
    /* Start with the empty list */
    struct node* head = NULL; 
    int i; 
  
    for (i = 5; i > 0; i--) { 
        push(&head, i); 
        printList(head); 
        printMiddle(head); 
    } 
  
    return 0; 
}

Java

class GFG{ 
  
static Node head; 
  
// Link list node 
class Node 
{ 
    int data; 
    Node next; 
  
    // Constructor 
    public Node(Node next, int data) 
    { 
        this.data = data; 
        this.next = next; 
    } 
} 
  
// Function to get the middle of  
// the linked list 
void printMiddle(Node head) 
{ 
    int count = 0; 
    Node mid = head; 
  
    while (head != null) 
    { 
  
        // Update mid, when 'count'  
        // is odd number  
        if ((count % 2) == 1) 
            mid = mid.next; 
  
        ++count; 
        head = head.next; 
    } 
  
    // If empty list is provided  
    if (mid != null) 
        System.out.println("The middle element is [" + 
                            mid.data + "]\n"); 
} 
  
void push(Node head_ref, int new_data) 
{ 
      
    // Allocate node  
    Node new_node = new Node(head_ref, new_data); 
  
    // Move the head to point to the new node  
    head = new_node; 
} 
  
// A utility function to print a  
// given linked list 
void printList(Node head) 
{ 
    while (head != null) 
    { 
        System.out.print(head.data + "-> "); 
        head = head.next; 
    } 
    System.out.println("null"); 
} 
  
// Driver code 
public static void main(String[] args) 
{ 
    GFG ll = new GFG(); 
  
    for(int i = 5; i > 0; i--)  
    { 
        ll.push(head, i); 
        ll.printList(head); 
        ll.printMiddle(head); 
    } 
} 
} 
  
// This code is contributed by mark_3

Output

5->NULL
The middle element is [5]

4->5->NULL
The middle element is [5]

3->4->5->NULL
The middle element is [4]

2->3->4->5->NULL
The middle element is [4]

1->2->3->4->5->NULL
The middle element is [3]

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