Added basic graph implementation challenge.

graphs_trees/graph/graph.py

@@ -0,0 +1,34 @@
+# Python 2 users: Run pip install enum34
+from enum import Enum
+
+
+class State(Enum):
+    unvisited = 1
+    visited = 2
+    visiting = 3
+
+
+class Node:
+
+    def __init__(self, id):
+        self.id = id
+        self.state = State.unvisited
+        self.connections = {}
+
+
+class Graph:
+
+    def __init__(self):
+        self.nodes = {}
+
+    def add_node(self, id):
+        node = Node(id)
+        self.nodes[id] = node
+        return node
+
+    def add_edge(self, source, dest, weight=0):
+        if source not in self.nodes:
+            self.add_node(source)
+        if dest not in self.nodes:
+            self.add_node(dest)
+        self.nodes[source].connections[self.nodes[dest]] = weight