fix

notes/Leetcode 题解 - 分治.md

@@ -77,6 +77,9 @@ The above output corresponds to the 5 unique BST's shown below:
 
 ```java
 public List<TreeNode> generateTrees(int n) {
+    if(n < 1){
+        return new LinkedList<TreeNode>();
+    }
     return generateSubtrees(1, n);
 }