Update 110_balanced_binary_tree.md

Author: Joseph Luce

leetcode/easy/110_balanced_binary_tree.md

@@ -20,8 +20,10 @@ class Solution:
             if root is None:
                 return 0
             left = balance_helper(root.left)
+            if left == -1:
+                return -1
             right = balance_helper(root.right)
-            if left == -1 or right == -1:
+            if right == -1:
                 return -1
             return max(left, right)+1 if abs(left-right) <= 1 else -1