In the process of analysis, the topics of arrangement and combination should be "not repeated or omitted". Therefore, we can use the method of "complete mutual exclusion" to classify and enumerate. The so-called "mutual exclusion" means that the listed combinations or arrangements are different and there is no overlap, while "complete" means that no possible combinations and arrangements are omitted. Take this problem as an example. Suppose five people are a, b, c, d, e,

First of all, all permutations and combinations fall into two categories:

1 and a are assigned values respectively.

2.a is assigned another 1 person.

In the case of 1, the other four people can only form teams in pairs. There are three combinations of C (4 4,2)/2 =1* *.

In the case of 2, it can be divided into three sub-categories, that is, Team A with C, D and E, so in each case, the remaining three people also have three combinations, so a * * * has 3x3=9 combinations.

So these five people are divided into three groups as required. A * * has 3+9= 12 combinations, and each combination needs to be arranged according to three schools, which is 3! =6, therefore, we can finally get 12x6=72 distribution methods.