max(2,4)=4min(15,max(2,4),6)=min(15,4,6)=4sum(10,min(15,max(2,4),6)))=sum(10,4)=14
sum(10,min(15,max(2,4),6))=14，先从最里层算起
函数=sum(10,min(15,max(2,1),3))=sum(10,min(15,2,3))=sum(10,2)=12选b
14
