Tree Degree
xThe degree is the number of children in the X node.
First order traversal: 1-> 2-> 4-> 5-> 3-> 6-> 7
In fact, it is very similar to deep search.
Sequential traversal: 4-> 2-> 5-> 1-> 6-> 3-> 7
Post ordinal traversal: 4-> 5-> 2-> 6-> 7-> 3-> 1