In addition to the complete arrangement of n, some sorting problems can be considered in combination with binding. For example, three people stand together in a fixed order, so these three people should consider it together. If three people don't stand together, we can regard it as a gap. For example, there are three people standing at 10, and their positions are fixed. So it must be the way to let these three people go first, and then let the fourth person go. Are there four ways to insert the gap, then the fifth person is five ways, and so on? I hope you can understand. Study hard and make progress every day!