数学パズル 緑 32 @ruby
def combination(a, b) factorial(a) / (factorial(b) * factorial(a-b)) end def factorial(n) ret = 1 (2..n).each do |i| ret *= i end ret end comb1 = [] N = 50 (2..N - 2).each do |i| comb1 << combination(14, i) end ans = 0 (2..N - 2).each do |i| ans += comb1[i - 2] * (i-1) * (N - i - 1) end puts ans