fn next_greater_elems<T: Copy + PartialOrd>(array: &[T]) -> Box<[Option<T>]> {
    let array_len = array.len();
    let mut stack = Vec::<usize>::with_capacity(array_len);
    let mut result = Vec::<Option<T>>::with_capacity(array_len);

    stack.push(0);
    result.push(None);

    for (index, element) in array.iter().enumerate().skip(1) {
        result.push(None);

        // Use "<=" for "Greater-Equal"
        while let Some(&stack_index) = stack.last().filter(|&&idx| array[idx] < *element) {
            result[stack_index] = Some(*element);
            stack.pop();
        }

        stack.push(index);
    }

    result.into_boxed_slice()
}