Add graph shortest path challenge

graphs_trees/graph_shortest_path/priority_queue.py

@@ -0,0 +1,41 @@
+import sys
+
+
+class PriorityQueueNode(object):
+
+    def __init__(self, obj, key):
+        self.obj = obj
+        self.key = key
+
+    def __repr__(self):
+        return str(self.obj) + ': ' + str(self.key)
+
+
+class PriorityQueue(object):
+
+    def __init__(self):
+        self.queue = []
+
+    def insert(self, node):
+        if node is not None:
+            self.queue.append(node)
+            return self.queue[-1]
+        return None
+
+    def extract_min(self):
+        if not self.queue:
+            return None
+        minimum = sys.maxsize
+        for index, node in enumerate(self.queue):
+            if node.key < minimum:
+                minimum = node.key
+                minimum_index = index
+        node = self.queue.pop(minimum_index)
+        return node.obj
+
+    def decrease_key(self, obj, new_key):
+        for node in self.queue:
+            if node.obj is obj:
+                node.key = new_key
+                return node
+        return None