269. Alien Dictionary

題目: There is a new alien language which uses the latin alphabet. However, the order among letters a ...

207. Course Schedule

題目： There are a total of n courses you have to take, labeled from 0 to n - 1. Some courses may hav ...

310. Minimum Height Trees

題目： For a undirected graph with tree characteristics, we can choose any node as the root. The resul ...