improve red black bst & fix typoes

Author: shellfly

README.md

@@ -20,7 +20,7 @@ pip install algs4
 ```
 
 ```python
-from algs4 import Stack
+from algs4.stack import Stack
 ```
 
 ## Index

algs4/edge_weighted_digraph.py

@@ -1,13 +1,13 @@
 """
- *  Execution:    python edge_weighted_graph.py filename.txt
+ *  Execution:    python edge_weighted_digraph.py filename.txt
  *  Data files:   https://algs4.cs.princeton.edu/43mst/tinyEWG.txt
  *                https://algs4.cs.princeton.edu/43mst/mediumEWG.txt
  *                https://algs4.cs.princeton.edu/43mst/largeEWG.txt
  *
- *  An edge-weighted undirected graph, implemented using adjacency lists.
+ *  An edge-weighted directed graph, implemented using adjacency lists.
  *  Parallel edges and self-loops are permitted.
  *
- *  % python edge_weighted_graph.py tinyEWD.txt
+ *  % python edge_weighted_digraph.py tinyEWD.txt
  * 8 vertices, 15 edges
  * 0: 0->2 0.26000 0->4 0.38000
  * 1: 1->3 0.29000
@@ -63,5 +63,5 @@ def edges(self):
 
 if __name__ == "__main__":
     import sys
-    graph = EdgeWeightedDiraph(file=open(sys.argv[1]))
+    graph = EdgeWeightedDigraph(file=open(sys.argv[1]))
     print(graph)

algs4/red_black_bst.py

@@ -339,7 +339,7 @@ def _delete_min(self, h):
         return self.balance(h)
 
     def balance(self, h):
-        if self.is_red(h):
+        if self.is_red(h) and not self.is_red(h.left):
             h = self.rotate_left(h)
         if self.is_red(h.left) and self.is_red(h.left.left):
             h = self.rotate_right(h)