365. Does the Graph Have a Cycle
Print Yes if the undirected graph contains a cycle, and No otherwise.
Walk the graph and watch for an edge that leads to a vertex already visited. That vertex closes a loop, so a cycle exists.
There is one thing to be careful about, and it is the whole problem. Every undirected edge appears in both adjacency lists, so from vertex 2 you will always see the edge back to the vertex you just came from. That is not a cycle. Carry the vertex you arrived from and skip it.
The graph may be disconnected, so start a traversal from every unvisited vertex, exactly as in problem 363.
Constraints - `1 ≤ n ≤ 100000` - `0 ≤ m ≤ 200000` - There are no self-loops and no repeated edges.
Input
The first line contains two integers n and m, the number of vertices and
edges. Each of the next m lines contains two integers u and v, an
undirected edge between those vertices. Vertices are numbered 1 to n.
Output
Print Yes or No on one line.
Hints
Four rungs, in order. The last two open once you have submitted an attempt. A wrong one counts.