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++) {
        // Get the original value even if it was modified
        int index = arr[i] % n;

        // If the value at that index is >= n, it means we've visited it
        if (arr[index] >= n) {
            return index;
        }

        // Mark the index as visited by adding n
        arr[index] += n;
    }
    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