PD com combinações em ruby

counting.rb

$comb = Hash.new do |comb, n|
    if n == 0
        comb[n] = Hash.new(0)
        comb[0][0] = 1;
    else
        comb[n] = Hash.new do |hash, k|
            if k == 0
                hash[k] = 1;
            else
                hash[k] = comb[n-1][k-1] + comb[n-1][k];
            end
        end
    end
end

puts $comb[100][3]