Three Hundred - Thirty Commit: Implement vertex_count() function

Author: AAdewunmi

Section_12(Graphs)/(1)_graph.py

@@ -58,4 +58,7 @@ def remove_edge(self, u, v):
         self._adjacent_matrix[u][v] = 0
 
     def exist_edge(self, u, v):
-        return self._adjacent_matrix[u][v] != 0
+        return self._adjacent_matrix[u][v] != 0
+
+    def vertex_count(self):
+        return self._vertices