diff --git a/include/s0235_lowest_common_ancestor_of_a_binary_search_tree.hpp b/include/s0235_lowest_common_ancestor_of_a_binary_search_tree.hpp
new file mode 100644
index 0000000..341f20e
--- /dev/null
+++ b/include/s0235_lowest_common_ancestor_of_a_binary_search_tree.hpp
@@ -0,0 +1,11 @@
+#ifndef S0235_LOWEST_COMMON_ANCESTOR_OF_A_BINARY_SEARCH_TREE_HPP
+#define S0235_LOWEST_COMMON_ANCESTOR_OF_A_BINARY_SEARCH_TREE_HPP
+
+#include "structures.hpp"
+
+class S0235 {
+ public:
+  TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q);
+};
+
+#endif
diff --git a/notes/src/SUMMARY.md b/notes/src/SUMMARY.md
index aa81aa1..fb41751 100644
--- a/notes/src/SUMMARY.md
+++ b/notes/src/SUMMARY.md
@@ -36,6 +36,7 @@
 
 - [理论基础](./btree_basic.md)
 - [遍历](./btree_iter.md)
+- [二叉搜索树](./bstree.md)
 
 # STL
 
diff --git a/notes/src/bstree.md b/notes/src/bstree.md
new file mode 100644
index 0000000..1bb9703
--- /dev/null
+++ b/notes/src/bstree.md
@@ -0,0 +1,3 @@
+# 二叉搜索树
+
+- [s0235](https://leetcode.cn/problems/lowest-common-ancestor-of-a-binary-search-tree/description/): 找两个指定节点的最近公共祖先。思路很简单，只要出现分岔（即一个在当前节点的左边，一个在当前节点的右边），那么这个分岔点就是最近公共祖先。
diff --git a/src/s0235_lowest_common_ancestor_of_a_binary_search_tree.cpp b/src/s0235_lowest_common_ancestor_of_a_binary_search_tree.cpp
new file mode 100644
index 0000000..91b5beb
--- /dev/null
+++ b/src/s0235_lowest_common_ancestor_of_a_binary_search_tree.cpp
@@ -0,0 +1,15 @@
+#include "s0235_lowest_common_ancestor_of_a_binary_search_tree.hpp"
+
+TreeNode* S0235::lowestCommonAncestor(TreeNode* root, TreeNode* p,
+                                      TreeNode* q) {
+  TreeNode* result = root;
+  while (!((p->val <= result->val && q->val >= result->val) ||
+           (q->val <= result->val && p->val >= result->val))) {
+    if (p->val < result->val) {
+      result = result->left;
+    } else {
+      result = result->right;
+    }
+  }
+  return result;
+}