Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>

int find_duplicate(int arr[], int n)
{
    for (int i = 0; i < n; i++)
    {
        int index = arr[i] % n;

        if (arr[index] >= n)
        {
            return index;
        }

        arr[index] += n;
    }

    return -1; // no duplicate (shouldn't happen if guaranteed)
}

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