Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>

int find_duplicate(int arr[], int n) {
    int freq[10] = {0}; // extra array to count occurrences

    // Count frequency
    for (int i = 0; i < n; i++) {
        freq[arr[i]]++;
    }

    // Find duplicate
    for (int i = 0; i < n; i++) {
        if (freq[i] > 1) {
            return i;   // return the duplicate number
        }
    }

    return -1; // if no duplicate found (should not happen)
}

int main() {
    int n;
    scanf("%d", &n);

    int arr[100];
    for (int i = 0; i < n; i++)
        scanf("%d", &arr[i]);

    int result = find_duplicate(arr, n);
    printf("%d", result);

    return 0;
}

Solving Approach