Tree
Basic concept
![](images/tree_relations.png)
(The above image is from http://alrightchiu.github.io/SecondRound/.)
Tree Definition:
Only one root.
No cycle exists.
Degree: For a given node, its number of children.
External Node / leaf Node: Any node that does not have child nodes.
Internal Node : Any node of a tree that has at least one child node.
Height of node: The biggest number of edges from one node to a leaf node.
![](images/tree_structure.png)
(The above image is from http://alrightchiu.github.io/SecondRound/.)