Find Duplicate in Range 0 and n-1

Code

#include <stdio.h>

int find_duplicate(int arr[], int n) {
    int retVal = -1;
    for (int i = 0; i<n-1;i++)
    {
        for (int j = i+1; j<n;j++)
        {
            if (arr[j] == arr[i])
            {
                retVal = arr[i];
                break;
            }
        }
    }
    return retVal;
}

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