Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>

int find_duplicate(int arr[], int n) {
    int i, j;
    for(i = 0; i < n; i++){
        // Key = arr[i]
        for(j = 1; j < (n - 1); j++){
            if(arr[i] == arr[(i + j) % n]){
                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