@author jackzhenguo
@desc
@date 2019/3/4返回一个set对象，集合内不允许有重复元素：
In [1]: a = [1,4,2,3,1]
In [2]: set(a)
Out[2]: {1, 2, 3, 4}
In [3]: b = set(a)
In [4]: b.add(5)
In [5]: b
Out[5]: {1, 2, 3, 4, 5}
In [6]: b.pop()
Out[6]: 1
In [7]: b
Out[7]: {2, 3, 4, 5}
In [8]: b.pop()
Out[8]: 2
In [9]: b
Out[9]: {3, 4, 5}
# 注意pop删除集合内任意一个元素
In [10]: help(b.pop)
Help on built-in function pop:
pop(...) method of builtins.set instance
Remove and return an arbitrary set element.
Raises KeyError if the set is empty.