58.c

// Find Middle Element

#include <stdio.h>
#include <stdlib.h>

struct Node {
    int data;
    struct Node* next;
};

struct Node* createNode(int value) {
    struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
    newNode->data = value;
    newNode->next = NULL;
    return newNode;
}

struct Node* insertAtEnd(struct Node* head, int value) {
    struct Node* newNode = createNode(value);

    if (head == NULL) return newNode;

    struct Node* temp = head;
    while (temp->next != NULL)
        temp = temp->next;

    temp->next = newNode;
    return head;
}

void display(struct Node* head) {
    while (head != NULL) {
        printf("%d -> ", head->data);
        head = head->next;
    }
    printf("NULL\n");
}

struct Node* findMiddle(struct Node* head) {
    struct Node* slow = head;
    struct Node* fast = head;

    while (fast != NULL && fast->next != NULL) {
        slow = slow->next;
        fast = fast->next->next;
    }

    return slow;
}

int main() {
    struct Node* head = NULL;

    head = insertAtEnd(head, 10);
    head = insertAtEnd(head, 20);
    head = insertAtEnd(head, 30);
    head = insertAtEnd(head, 40);

    display(head);

    struct Node* mid = findMiddle(head);
    printf("Middle Element: %d\n", mid->data);

    return 0;
}