【Example 3】How many four-digit numbers without repeated digits can be formed from the digits $1,2,3,4,5,6,7,8,9$?
Solution: Since we need to select 4 different elements in sequence from $1,2, \cdots, 9$ to serve as the thousands, hundreds, tens, and units digits respectively, this is a permutation problem, with a total of
$$
A_{9}^{4}=9 \times 8 \times 7 \times 6=3024
$$

different ways, which means 3024 different four-digit numbers without repeated digits can be formed.

In problems involving "no repeated digits," they generally belong to permutation problems and can therefore be directly reduced to permutation problems for solving. Example 3 is a basic case. Below, we will look at two more complex examples. Example 4 shares some similarities with Example 7 from the first section, but due to one being "no repeated digits" and the other being "repeated digits," the handling methods are quite different. Additionally, in the solution technique of Example 5, there are certain points worth the reader's attention, which should be noted.