最近筆試的時候遇到這個問題，特來mark一下以便自己回憶。(http://img.blog.csdn.net/20170305190141929?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvTU5KTEpfMjNOTUs=/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast)