"간선을 하나 지웠을 때 그래프가 더 이상 연결되지 않으면 트리가 아니다. 간선을 하나 추가했을 때 사이클이 생기면 트리가 아니다." 라고 되어 있는데... 오히려 그 반대죠.

Trees are the subset of graphs that have the following 3 properties:

1. It is connected: for every node you can reach every other node following edges.
2. If an edge is removed, the graph is no longer connected. That is, some nodes cannot be reached anymore.
3. When an edge is added between two existing nodes A and B, a cycle is created. There is a cycle if there is more than one way to go from A to B.