diff --git a/notes/37. 序列化二叉树.md b/notes/37. 序列化二叉树.md
index 2a4562b5..68ba5457 100644
--- a/notes/37. 序列化二叉树.md	
+++ b/notes/37. 序列化二叉树.md	
@@ -8,37 +8,57 @@
 
 ## 解题思路
 
+用前序遍历序列化，空节点写成 #，值之间用空格分隔。例如根为 1、左孩子为 2、右孩子 3 下面有 4 和 5 的树，序列化为：
+
+```text
+1 2 # # 3 4 # # 5 # #
+```
+
+<div align="center"> <img src="pics/offer37-serialize-tree.png" width="560px"/> </div><br>
+
+反序列化按同样的前序顺序读取：读到 # 返回 null，否则创建节点，再依次递归构建左子树和右子树。# 标出了每棵子树在哪里结束，因此只用前序序列就能还原出唯一的树；不记录空节点时，需要前序和中序两个序列（见 [7. 重建二叉树](7.%20重建二叉树.md)）。
+
+序列化时所有节点追加到同一个 StringBuilder 中，时间复杂度为 O(N)。如果每层递归都返回字符串再拼接，子树的结果会被反复复制，树退化成链时为 O(N<sup>2</sup>)。
+
 ```java
 class Codec {
 
-    private String deserializeStr;
+    private String[] tokens;
+    private int index;
 
     // Encodes a tree to a single string.
     public String serialize(TreeNode root) {
-        if (root == null) {
-            return "#";
+        StringBuilder sb = new StringBuilder();
+        serialize(root, sb);
+        return sb.toString();
+    }
+
+    private void serialize(TreeNode root, StringBuilder sb) {
+        if (sb.length() > 0) {
+            sb.append(' ');
         }
-        return root.val + " " + serialize(root.left) + " " + serialize(root.right);
+        if (root == null) {
+            sb.append('#');
+            return;
+        }
+        sb.append(root.val);
+        serialize(root.left, sb);
+        serialize(root.right, sb);
     }
 
     // Decodes your encoded data to tree.
     public TreeNode deserialize(String data) {
-        deserializeStr = data;
+        tokens = data.split(" ");
+        index = 0;
         return deserialize();
     }
 
     private TreeNode deserialize() {
-        if (deserializeStr.length() == 0) {
-            return null;
-        }
-        int index = deserializeStr.indexOf(" ");
-        String node = index == -1 ? deserializeStr : deserializeStr.substring(0, index);
-        deserializeStr = index == -1 ? "" : deserializeStr.substring(index + 1);
+        String node = tokens[index++];
         if (node.equals("#")) {
             return null;
         }
-        int val = Integer.parseInt(node);
-        TreeNode t = new TreeNode(val);
+        TreeNode t = new TreeNode(Integer.parseInt(node));
         t.left = deserialize();
         t.right = deserialize();
         return t;
diff --git a/notes/pics/offer37-serialize-tree.png b/notes/pics/offer37-serialize-tree.png
new file mode 100644
index 00000000..ac009ad4
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