Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>

int find_duplicate(int arr[], int n) {
    int i;
    for (i = 0; i < n; i++) {
        int index = arr[i];
        if (index >= n)
            index -= n;
        if (arr[index] >= n)
            return index;
        arr[index] += n; /* mark index */
    }
    return -1;
}

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