program that reads in a list of integers from a file and checks if it contains any duplicate elements using an efficient algorithm

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

#define HASH_SIZE 1000

struct ListNode {
  int val;
  struct ListNode *next;
};

struct HashTable {
  struct ListNode *table[HASH_SIZE];
};

struct HashTable *createHashTable() {
  struct HashTable *hashTable = malloc(sizeof(struct HashTable));
  for (int i = 0; i < HASH_SIZE; i++) {
    hashTable->table[i] = NULL;
  }
  return hashTable;
}

void insert(struct HashTable *hashTable, int val) {
  int index = val % HASH_SIZE;
  struct ListNode *node = malloc(sizeof(struct ListNode));
  node->val = val;
  node->next = hashTable->table[index];
  hashTable->table[index] = node;
}

bool find(struct HashTable *hashTable, int val) {
  int index = val % HASH_SIZE;
  struct ListNode *node = hashTable->table[index];
  while (node != NULL) {
    if (node->val == val) {
      return true;
    }
    node = node->next;
  }
  return false;
}

int main(int argc, char *argv[]) {
  if (argc < 2) {
    printf("Usage: %s filename\n", argv[0]);
    return 1;
  }

  char *filename = argv[1];
  FILE *file = fopen(filename, "r");
  if (file == NULL) {
    printf("Error opening file\n");
    return 1;
  }

  struct HashTable *hashTable = createHashTable();

  int val;
  while (fscanf(file, "%d", &val) == 1) {
    if (find(hashTable, val)) {
      printf("Duplicate element: %d\n", val);
      break;
    }
    insert(hashTable, val);
  }

  fclose(file);
  return 0;
}