http://www.zhihu.com/question/22744751/answer/22473212
http://www.zhihu.com/question/20334217
http://www.zhihu.com/question/20098032
http://www.zhihu.com/question/20551864
知乎 zhihu
原文：http://www.cnblogs.com/Badcode/p/zhihu.html
