Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>
#include <stdlib.h>

int compare_ints(const void *a, const void *b) {
    return (*(const int *)a - *(const int *)b);
}

int find_duplicate(int arr[], int n) {
    qsort(arr, n, sizeof(int), compare_ints);
    for(int i= 1;i <n; i++) {
        if(arr[i] == arr[i-1]) {
            return arr[i];
        }
    }
    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