function oddOccurrencesInArray(a: number[]): number | null {
  const count = new Map<number, number>();

  for (const value of a) {
    if (!count.has(value)) {
      count.set(value, 1);
    } else {
      count.delete(value);
    }
  }

  const first = count.keys().next();
  return first.done ? null : first.value;
}