"Unit" positioning, "Ten" positioning and exchange method in positioning methods. For example, 1, 2,3 form a two-digit number, and the ten digits and the single digits of each two-digit number cannot be the same.

The "unit" positioning method is to locate 1:2 1, 31in the unit; Position 2 in the unit:12,32; At the position of the unit 3:13,23.

way

The key to solving the problem of pendulum number is not to repeat or omit. You can use enumeration method, first consider the high position, then consider the low position, arrange in order, and list all possible numbers one by one.

When using the knowledge of combination to solve problems, we should first find out the possibility of combination by contact method or list method, and then answer. The sum of any two numbers is a collocation problem, which has nothing to do with the order. There must be a certain order in the arrangement and combination, so that it is not easy to miss.