Given a directed acyclic graph $D = (V,A)$, a vertex $v \in V$ is a source if its indegree is zero, meaning that it has only outgoing arcs.Does there exist a linear time algorithm to find a source in a given directed acyclic graph?Follow-up question:  Can one in linear time find all sources?
Finding a source of a directed acyclic graph in linear time
algorithms;graph theory