diff --git a/src/main/java/leetcode/editor/cn/LegalBinarySearchTreeLcci.java b/src/main/java/leetcode/editor/cn/LegalBinarySearchTreeLcci.java
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+//<p>实现一个函数，检查一棵二叉树是否为二叉搜索树。</p><strong>示例 1:</strong><pre><strong>输入:</strong><br>    2<br>   / &#92<br>  1   3<br><strong>输出:</strong> true<br></pre><strong>示例 2:</strong><pre><strong>输入:</strong><br>    5<br>   / &#92<br>  1   4<br>     / &#92<br>    3   6<br><strong>输出:</strong> false<br><strong>解释:</strong> 输入为: [5,1,4,null,null,3,6]。<br>     根节点的值为 5 ，但是其右子节点值为 4 。</pre><div><div>Related Topics</div><div><li>树</li><li>深度优先搜索</li><li>二叉搜索树</li><li>二叉树</li></div></div><br><div><li>👍 76</li><li>👎 0</li></div>
+package leetcode.editor.cn;
+
+import com.code.leet.entiy.TreeNode;
+
+// 面试题 04.05:合法二叉搜索树
+public class LegalBinarySearchTreeLcci {
+    public static void main(String[] args) {
+        Solution solution = new LegalBinarySearchTreeLcci().new Solution();
+        // TO TEST
+    }
+    //leetcode submit region begin(Prohibit modification and deletion)
+
+    /**
+     * Definition for a binary tree node.
+     * public class TreeNode {
+     * int val;
+     * TreeNode left;
+     * TreeNode right;
+     * TreeNode(int x) { val = x; }
+     * }
+     */
+    class Solution {
+        public boolean isValidBST(TreeNode root) {
+            return dfs(root, null, null);
+        }
+
+        public boolean dfs(TreeNode node, Integer lower, Integer upper) {
+            if (node == null) {
+                return true;
+            }
+
+            int val = node.val;
+            if (lower != null && val <= lower) {
+                return false;
+            }
+            if (upper != null && val >= upper) {
+                return false;
+            }
+
+            if (!dfs(node.right, val, upper)) {
+                return false;
+            }
+            return dfs(node.left, lower, val);
+        }
+    }
+//leetcode submit region end(Prohibit modification and deletion)
+
+}
diff --git a/src/main/java/leetcode/editor/cn/doc/content/LegalBinarySearchTreeLcci.md b/src/main/java/leetcode/editor/cn/doc/content/LegalBinarySearchTreeLcci.md
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index 0000000..f7fecfc
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@@ -0,0 +1 @@
+<p>实现一个函数，检查一棵二叉树是否为二叉搜索树。</p><strong>示例 1:</strong><pre><strong>输入:</strong><br>    2<br>   / &#92<br>  1   3<br><strong>输出:</strong> true<br></pre><strong>示例 2:</strong><pre><strong>输入:</strong><br>    5<br>   / &#92<br>  1   4<br>     / &#92<br>    3   6<br><strong>输出:</strong> false<br><strong>解释:</strong> 输入为: [5,1,4,null,null,3,6]。<br>     根节点的值为 5 ，但是其右子节点值为 4 。</pre><div><div>Related Topics</div><div><li>树</li><li>深度优先搜索</li><li>二叉搜索树</li><li>二叉树</li></div></div><br><div><li>👍 76</li><li>👎 0</li></div>
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