基本原理要弄清 ,分类分步好区分 .特殊元位打头阵 ,插空捆绑间相邻 .正反两面方法并 ,相互验证结论真 .常见问题多留心 ,有的问题构模型 .解释 :加法原理和乘法原理是解排列组合问题的基础 ,只有深刻理解才能正确区分是分类还是分步 .对题目中出现的特殊元素和特殊位置一般要 The basic principle must be clarified, and the classification should be divided into different steps. The special element should be the first one, and the empty string should be adjacent to each other. The positive and negative methods should be used together to verify the conclusions of each other. The common question should be careful, and some problems must be modeled. Explanation: The principle of addition The principle of multiplication and multiplication is the basis for solving the problem of arranging and combining. Only deep understanding can correctly distinguish whether it is a classification or a step. It is generally necessary to have special elements and special positions in the title.