class Array
    def sum
        ans = self.inject{|ac, n| ac += n}
        ans ? ans : 0
    end

    def square
        map{|n| n * n}
    end

    def square!
        ans = []
        for i in 1 .. length - 1
            self[i] = self[i] * self[i]
        end
    end
end