从网站上搜寻到的比较好的解释
网址为 http://www.mathsisfun.com/sets/injective-surjective-bijective.html
Function: No element in A is left.
One-to-many is not OK.
Injective:
Many-to-one is not OK.
Subjective:
No element in B is left.
Bijective:
One-to-one perfect correspondence.
