add is_empty check for bst

Author: shellfly

algs4/bst.py

@@ -113,6 +113,8 @@ def _keys(self, x, queue, lo, hi):
             self._keys(x.right, queue, lo, hi)
 
     def max(self):
+        if self.is_empty():
+            raise Exception("empty bst")
         return self._max(self.root).key
 
     def _max(self, x):
@@ -122,6 +124,8 @@ def _max(self, x):
         return self._max(x.right)
 
     def min(self):
+        if self.is_empty():
+            raise Exception("empty bst")
         return self._min(self.root).key
 
     def _min(self, x):
@@ -229,6 +233,10 @@ def _delete_min(self, x):
         x.N = self._size(x.left) + self._size(x.right) + 1
         return x
 
+    def is_empty(self):
+        return self.root == None
+
+
 if __name__ == '__main__':
     import sys