381. How Much Can You Reach
The edges are directed. Print how many vertices can be reached from vertex 1, counting vertex 1 itself.
The traversal is the one from problem 362, unchanged. What changes is that following an edge is now one-way, so reachability stops being symmetric: 1 may reach 5 while 5 cannot reach 1.
A consequence worth noticing in the tests: two vertices in the same undirected component can be unreachable from each other once the arrows are put back.
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
directed edges. Each of the next m lines contains two integers u and v,
a directed edge from u to v and NOT from v to u. Vertices are numbered
1 to n.
Output
Print one integer, the number of vertices reachable from vertex 1.
Hints
Four rungs, in order. The last two open once you have submitted an attempt. A wrong one counts.