Find Duplicate in Range 0 and n-1

Code

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

int compare_ints(const void *a, const void *b) {
    int arg1 = *(const int *)a;
    int arg2 = *(const int *)b;
    return arg1 - arg2; // Returns negative if arg1 < arg2, positive if arg1 > arg2, zero if equal
}

int find_duplicate(int arr[], int n) {
    // Your logic here
    int lastdup = -1;
    qsort(arr, n, sizeof(int), compare_ints);
    for(int i=0; i<n-1; i++)
    {
        if((arr[i] == arr[i+1]) && lastdup != arr[i])
            lastdup = arr[i];
    }
    return lastdup;
}

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