C Program to Find Middle Node of a Linked List

• Write a C program to print alternate nodes of given linked list.
• Function to print alternate nodes of a linked list.

Given a singly linked list, we have to find the middle node of given linked list. Let the length of linked list be N. Middle node of linked list will be (N/2 + 1)^th node from beginning. For Example:

Input Linked List
1-->2-->3-->4-->5-->6-->7
Length of Linked List : 7
Middle Node :
4

Input Linked List
1-->2-->3-->4-->5-->6-->7-->8
Length of Linked List : 8
Middle Node :
5

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Method 1

Find middle node of a linked list using slow and fast pointer.

Algorithm to print middle node of linked list
Let "head" be the head pointer of given linked list.

• We will use two pointers "front" and "back" pointer. Initially, set both pointer to head node.
• Using a loop, traverse linked list until fast pointer reached last node of linked list.(fast != NULL && fast->next != NULL)
• In every iteration, slow pointer will move one node whereas fast pointer will move two node.
• When fast pointer reaches last node then slow pointer will be pointing to middle node.

In this program, we will use a user defined function "printMiddleNode" which takes head node of a linked list as input and print middle node by implementing above mentioned algorithm.

void printMiddleNode(struct node *head){
    /* Input Validation */
    if(head == NULL){
        printf("Error : Invalid Input !!!!\n");
        return INT_MIN;
    }
    struct node *slow, *fast;
    slow = fast = head;

    while(fast != NULL && fast->next != NULL) {
        fast = fast->next->next;
        slow = slow->next;
    }
    
    printf("\nMiddle Node : %d\n", slow->data);
}

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C program to print middle node of a linked list.

#include <stdio.h>
#include <stdlib.h>
 
/* A structure of linked list node */
struct node {
  int data;
  struct node *next;
} *head;

void initialize(){
    head = NULL;
}

/* 
Given a Inserts a node in front of a singly linked list. 
*/
void insert(int num) {
    /* Create a new Linked List node */
    struct node* newNode = (struct node*) malloc(sizeof(struct node));
    newNode->data  = num;
    /* Next pointer of new node will point to head node of linked list  */
    newNode->next = head;
    /* make new node as new head of linked list */
    head = newNode;
    printf("Inserted Element : %d\n", num);
}

void printMiddleNode(struct node *head){
    /* Input Validation */
    if(head == NULL){
        printf("Error : Invalid Input !!!!\n");
        return INT_MIN;
    }
    struct node *slow, *fast;
    slow = fast = head;
    /* In every iteration, slow pointer will move one nede whereas
     fast pointer will move two node. When fast pointer reaches 
     last node then slow pointer will be pointing to middle node */
    while(fast != NULL && fast->next != NULL) {
        fast = fast->next->next;
        slow = slow->next;
    }
    
    printf("\nMiddle Node : %d\n", slow->data);
}

/*
Prints a linked list from head node till tail node 
*/
void printLinkedList(struct node *nodePtr) {
  while (nodePtr != NULL) {
     printf("%d", nodePtr->data);
     nodePtr = nodePtr->next;
     if(nodePtr != NULL)
         printf("-->");
  }
}
 
int main() {
    initialize();
    /* Creating a linked List*/
    insert(3);  
    insert(7); 
    insert(12); 
    insert(5);
    insert(9);
    
    printf("\nLinked List\n");
    printLinkedList(head);
    
    /* Printing Middle Node of Linked List */
    printMiddleNode(head);
    return 0;
}

Output

Inserted Element : 3
Inserted Element : 7
Inserted Element : 12
Inserted Element : 5
Inserted Element : 9

Linked List
9-->5-->12-->7-->3
Middle Node : 12

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Method 1

Find middle node of a linked list by counting nodes in linked list.

Algorithm to print middle node of linked list
Let "head" be the head pointer of given linked list.

• Traverse the linked list and count total number of nodes in linked list. Let it be LENGTH.
• Now, traverse the linked list again till LENGTH/2 +1 node and print (LENGTH/2 +1)^th