Find Duplicate in Range 0 and n-1

#include <stdio.h>

int find_duplicate(int arr[], int n) {
    int r = 0;
    for (int i = 0; i < n; i++) {
        while (arr[i] != arr[arr[i]]) {
            int tmp = arr[i];
            arr[i] = arr[tmp];
            arr[tmp] = tmp;
        }
    }

    for (int i = 0; i < n; i++) {
        if (arr[i] != i) return arr[i];
    }
    return 0;
}

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;
}