diff --git a/README.md b/README.md
index f328996..8aaf808 100644
--- a/README.md
+++ b/README.md
@@ -23,6 +23,7 @@
 |剑指 Offer 26|[树的子结构](#)|[Java](./src/com/leetcode/submissions/IsSubStructure.java)|Medium|
 |剑指 Offer 55 - II|[平衡二叉树](#)|[Java](./src/com/leetcode/submissions/IsBalanced.java)|Easy|
 |814 |[二叉树剪枝](#)|[Java](./src/com/leetcode/submissions/BinaryTreePruning.java)|Medium|
+|面试题 04.05|[合法二叉搜索树](#)|[Java](./src/com/leetcode/submissions/IsValidBST.java)|Medium|
 
 
 ## 题目目录
diff --git a/src/com/leetcode/submissions/IsValidBST.java b/src/com/leetcode/submissions/IsValidBST.java
new file mode 100644
index 0000000..b67e5ef
--- /dev/null
+++ b/src/com/leetcode/submissions/IsValidBST.java
@@ -0,0 +1,28 @@
+package com.leetcode.submissions;
+
+import com.leetcode.extend.TreeNode;
+
+/**
+ * 面试题 04.05. 合法二叉搜索树
+ * Create by Ranzd on 2021-01-25 20:18
+ *
+ * @author ranzhendong@maoyan.com
+ */
+public class IsValidBST {
+    public long MAX_VAL = Long.MIN_VALUE;
+
+    public boolean isValidBST(TreeNode root) {
+        if (root == null) {
+            return true;
+        }
+        boolean left = isValidBST(root.left);
+        if (!left) {
+            return false;
+        }
+        if (MAX_VAL >= root.val) {
+            return false;
+        }
+        MAX_VAL = root.val;
+        return isValidBST(root.right);
+    }
+}