improve the answer of exercise 3.37

I only improved the exercise 3.37 but the editor told me that I changed much, I don't know why. The first commit   is wrong, so I have done a revert.  Please check the exercise 3.37.
PS. The grammar of readme.md also makes trouble ...o(╯□╰)o
awesome! @evan617  I guess you have coding experience using other languages (such as C lang). Am I right? :smile: