1, enumeration method: suitable for small-scale problems that can enumerate all situations. By listing all possible situations one by one, and then screening or judging, the final result is obtained. The disadvantage is that when the problem is big, the time complexity will be high.
3. Plug-in method: it is suitable for large-scale problems, and it is difficult to list all the situations directly. The method is to put several identical items into several different boxes, so that there is at least one item in each box. Its advantage is that it can quickly solve problems such as the number of combinations. Generally speaking, enumeration method and plug-in method have their own types of problems that they are good at solving. In practical application, we need to choose which method to use according to the specific situation.