1. If flag does not contain 4, the address of the next node must be given, or the next address does not exist (ie, the end state)
If it contains 4, then the position where the current node ends is the next address
2. 2 means the current node data has ended
3. Data multiplexing, only the tail data can be multiplexed, and the content of the entire node must be consistent (multi-node data multiplexing can be done)
4. The position node where S starts and finally do compile
5.
