refer to:
http://www.cnblogs.com/icejoywoo/p/3531869.html
对于python3, 可能有不一样之处, refer to:
http://docspy3zh.readthedocs.io/en/latest/tutorial/datastructures.html
dict 几种遍历方式的效率差比别:
http://blog.csdn.net/cugbabybear/article/details/25037687