排列组合(计数)问题是自主招生考试中的热点问题.同学们在遇到这类问题时,要学会转化条件,提炼出基本的模型、方法,从而事半功倍地根据一个问题的解决学会一类问题的解决.实际上,教材中介绍的加法原理、乘法原理以及排列数公式、组合数公式就是可以用来解决计数这一大类问题的最基本的模型、方法. The problem of permutation and combination (counting) is a hot topic in the independent admissions examination. When students encounter such problems, they must learn to transform conditions and refine basic models and methods, so that they can learn a type of problem based on the solution of a problem. In fact, the addition principle, multiplication principle, arrangement number formula, and combination number formula introduced in the textbook are the most basic models and methods that can be used to solve the problem of counting.