Add docstr and algorithm to BFS shortest path module (TheAlgorithms#1637)

* Add docs and type alias to bfs_shortest_path.py

* Add bfs_shortest_path_distance algorithm

* Make requested changes

Author: kylepw

graphs/bfs_shortest_path.py

@@ -1,3 +1,11 @@
+"""Breadth-first search shortest path implementations.
+
+    doctest:
+    python -m doctest -v bfs_shortest_path.py
+
+    Manual test:
+    python bfs_shortest_path.py
+"""
 graph = {
     "A": ["B", "C", "E"],
     "B": ["A", "D", "E"],
@@ -9,7 +17,22 @@
 }
 
 
-def bfs_shortest_path(graph, start, goal):
+def bfs_shortest_path(graph: dict, start, goal) -> str:
+    """Find shortest path between `start` and `goal` nodes.
+
+        Args:
+            graph (dict): node/list of neighboring nodes key/value pairs.
+            start: start node.
+            goal: target node.
+
+        Returns:
+            Shortest path between `start` and `goal` nodes as a string of nodes.
+            'Not found' string if no path found.
+
+        Example:
+            >>> bfs_shortest_path(graph, "G", "D")
+            ['G', 'C', 'A', 'B', 'D']
+    """
     # keep track of explored nodes
     explored = []
     # keep track of all the paths to be checked
@@ -44,4 +67,48 @@ def bfs_shortest_path(graph, start, goal):
     return "So sorry, but a connecting path doesn't exist :("
 
 
-bfs_shortest_path(graph, "G", "D")  # returns ['G', 'C', 'A', 'B', 'D']
+def bfs_shortest_path_distance(graph: dict, start, target) -> int:
+    """Find shortest path distance between `start` and `target` nodes.
+
+        Args:
+            graph: node/list of neighboring nodes key/value pairs.
+            start: node to start search from.
+            target: node to search for.
+
+        Returns:
+            Number of edges in shortest path between `start` and `target` nodes.
+            -1 if no path exists.
+
+        Example:
+            >>> bfs_shortest_path_distance(graph, "G", "D")
+            4
+            >>> bfs_shortest_path_distance(graph, "A", "A")
+            0
+            >>> bfs_shortest_path_distance(graph, "A", "H")
+            -1
+    """
+    if not graph or start not in graph or target not in graph:
+        return -1
+    if start == target:
+        return 0
+    queue = [start]
+    visited = [start]
+    # Keep tab on distances from `start` node.
+    dist = {start: 0, target: -1}
+    while queue:
+        node = queue.pop(0)
+        if node == target:
+            dist[target] = (
+                dist[node] if dist[target] == -1 else min(dist[target], dist[node])
+            )
+        for adjacent in graph[node]:
+            if adjacent not in visited:
+                visited.append(adjacent)
+                queue.append(adjacent)
+                dist[adjacent] = dist[node] + 1
+    return dist[target]
+
+
+if __name__ == "__main__":
+    print(bfs_shortest_path(graph, "G", "D"))  # returns ['G', 'C', 'A', 'B', 'D']
+    print(bfs_shortest_path_distance(graph, "G", "D"))  # returns 4