Add Factorial in Crystal (#4487)

archive/c/crystal/factorial.cr

@@ -0,0 +1,29 @@
+begin
+  if ARGV.empty?
+    raise "Usage: please input a non-negative integer"
+  end
+
+  input = ARGV[0].to_i?
+
+  if input.nil?
+    raise "Usage: please input a non-negative integer"
+  end
+
+  input = input.not_nil!
+
+  if input >= 0 && input <= 12
+    puts factorial(input)
+  elsif input > 12
+    raise "#{input} is out of the reasonable bounds for calculation."
+  else
+    raise "Usage: please input a non-negative integer"
+  end
+
+rescue e
+  puts e.message
+end
+
+def factorial(n)
+  return 1 if n == 0
+  n * factorial(n - 1)
+end