一、合理分类,准确分步解含有约束条件的排列组合问题,应按元素的性质进行分类,分类时需要满足两个条件:(1)类与类之间要互斥(保证不重复);(2)总数要完备(保证不遗漏).要确定一个合理的分类标准,应按事件发生的连贯过程进行分步,分步时必须做到步与步之间互相独立,互不干扰,并确保具有连续性. First, the reasonable classification, accurately step by step with the constraints of the permutation and combination of the elements should be classified according to the nature of the classification need to meet two conditions: (1) classes and classes to be mutually exclusive (guaranteed not to repeat); (2) The total number should be complete (to ensure no omission) To determine a reasonable taxonomic criteria, the process should be divided according to the coherent process of events, and the step-by-step steps must be independent and do not interfere with each other. Ensure continuity.