fn sockMerchant(n: i32, ar: &[i32]) -> i32 {
    let odd_socks = ar.iter().fold(HashSet::new(), |mut acc, sock| {
        if acc.remove(sock) == false {
            acc.insert(sock);
        }
        acc
    })
    .len() as i32;
    
    (n - odd_socks) / 2
}