Challenge: implement enumerable methods using #inject instead of #each.

injections.rb

def collect(&block)
  inject([]) { |memo, obj| memo << block.call(obj) }
end

def detect(&block)
  inject(nil) { |memo, obj| memo ||= obj if block.call(obj) }
end

def reject(&block)
  inject([]) { |memo, obj| block.call(obj) ? memo : memo << obj }
end

def select(&block)
  inject([]) { |memo, obj| block.call (obj) ? memo << obj : memo }
end