public int numSinks() {
		int i, cnt = 0;

		for (i = 0; i < nextIndex; i++) {
			if ((firstOutgoingEdge[i] == null) && (vertices[i] != null)) {
				cnt++;
			}
		}
		return cnt;
	}
Return the number of sinks.  
This equals the number of vertices with no outgoing
edges in the VertexSet.
Ghidra/Framework/Generic/src/main/java/ghidra/util/graph/VertexSet.java