Given an oriented graph, return true if paths have a specified length
Let's say a vertex $v$ is at distance $t$ from the set $A$ if there exists a path of length $t$ from some element of $A$ to $v$; and there is no shorter path.

Can you think of an efficient way to compute the distance from $A$ to each individual vertex in the graph?

Perhaps you might like to modify BFS.  Hint:


   Maybe you can think of some way to change the initialization, to make BFS do what you want.