From 6001fe179c63a9a18546de787d563adfcedbc79c Mon Sep 17 00:00:00 2001
From: shellfly <shell0fly@gmail.com>
Date: Fri, 31 Jan 2020 20:35:48 +0800
Subject: [PATCH] add is_empty check for bst

---
 algs4/bst.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/algs4/bst.py b/algs4/bst.py
index 041ad5c..46cdc70 100644
--- a/algs4/bst.py
+++ b/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