diff --git a/notes/Leetcode 题解 - 树.md b/notes/Leetcode 题解 - 树.md
index 410553f7..c0243939 100644
--- a/notes/Leetcode 题解 - 树.md	
+++ b/notes/Leetcode 题解 - 树.md	
@@ -427,13 +427,18 @@ Maximum amount of money the thief can rob = 3 + 3 + 1 = 7.
 ```
 
 ```java
+Map<TreeNode, Integer> cache = new HashMap<>(); 
+
 public int rob(TreeNode root) {
     if (root == null) return 0;
+    if (cache.containsKey(root)) return cache.get(root);
     int val1 = root.val;
     if (root.left != null) val1 += rob(root.left.left) + rob(root.left.right);
     if (root.right != null) val1 += rob(root.right.left) + rob(root.right.right);
     int val2 = rob(root.left) + rob(root.right);
-    return Math.max(val1, val2);
+    int res = Math.max(val1, val2);
+    cache.put(root, res);
+    return res;
 }
 ```