use std::collections::HashMap;

fn odd_occurrences_in_array(a: &[i64]) -> Option<i64> {
    let mut count: HashMap<i64, bool> = HashMap::new();
    for &value in a {
        if count.contains_key(&value) {
            count.remove(&value);
        } else {
            count.insert(value, true);
        }
    }

    count.keys().next().copied()
}