Xiaoqiu 123456

6C2 means that the numbers of the two balls are the same as those of the box.

The number of the remaining 4 balls and 4 boxes cannot be the same! ! !

By enumeration, we can know that there are 9 different schemes.

Then multiply by: the answer is 135.

Attachment: There cannot be two schemes for three balls and three container numbers.

Four balls and four case numbers cannot have nine schemes.

Three balls and three case numbers can't have 29 schemes (I don't remember this clearly)

These are either self-listed or memorized, but not many.