diff --git a/OJ notes/OJ-index.md b/OJ notes/OJ-index.md
index ae919e0..c1afdcb 100644
--- a/OJ notes/OJ-index.md	
+++ b/OJ notes/OJ-index.md	
@@ -59,6 +59,7 @@ tag:#leetcode
 - [[Leetcode Intersection-of-Two-Arrays-II]]
 - [[Leetcode Invert-Binary-Tree]]
 - [[Leetcode Linked-List-Cycle]]
+- [[Leetcode Lowest-Common-Ancestor-Of-a-Binary-Search-Tree]]
 - [[Leetcode Maximum-Depth-Of-Binary-Tree]]
 - [[Leetcode Maximum-Difference-Between-Increasing-Elements]]
 - [[Leetcode Maxinum-subarray]]
@@ -114,6 +115,7 @@ tag:#DS tag:#coding_problem -tag:#template_remove_me
 - [[Leetcode Intersection-of-Two-Arrays-II]]
 - [[Leetcode Invert-Binary-Tree]]
 - [[Leetcode Linked-List-Cycle]]
+- [[Leetcode Lowest-Common-Ancestor-Of-a-Binary-Search-Tree]]
 - [[Leetcode Maximum-Depth-Of-Binary-Tree]]
 - [[Leetcode Maxinum-subarray]]
 - [[Leetcode Merge-Sorted-Array]]
@@ -167,6 +169,7 @@ tag:#algorithm tag:#coding_problem -tag:#template_remove_me
 - [[Leetcode Intersection-of-Two-Arrays-II]]
 - [[Leetcode Invert-Binary-Tree]]
 - [[Leetcode Linked-List-Cycle]]
+- [[Leetcode Lowest-Common-Ancestor-Of-a-Binary-Search-Tree]]
 - [[Leetcode Maximum-Depth-Of-Binary-Tree]]
 - [[Leetcode Maximum-Difference-Between-Increasing-Elements]]
 - [[Leetcode Maxinum-subarray]]
diff --git a/OJ notes/pages/Leetcode Lowest-Common-Ancestor-Of-a-Binary-Search-Tree.md b/OJ notes/pages/Leetcode Lowest-Common-Ancestor-Of-a-Binary-Search-Tree.md
new file mode 100644
index 0000000..1c8d754
--- /dev/null
+++ b/OJ notes/pages/Leetcode Lowest-Common-Ancestor-Of-a-Binary-Search-Tree.md	
@@ -0,0 +1,106 @@
+# Leetcode Lowest-Common-Ancestor-Of-a-Binary-Search-Tree
+
+#### 2022-07-08 11:53
+
+> ##### Algorithms:
+> #algorithm #binary_search #DFS 
+> ##### Data structures:
+> #DS #binary_tree #binary_search_tree 
+> ##### Difficulty:
+> #coding_problem #difficulty-easy 
+> ##### Additional tags:
+> #leetcode 
+> ##### Revisions:
+> N/A
+
+##### Related topics:
+```expander
+tag:#binary_search_tree
+```
+ 
+- [[Leetcode Two-Sum-IV-Input-Is-a-BST]]
+- [[Leetcode Validate-Binary-Search-Tree]]
+ 
+
+##### Links:
+- [Link to problem](https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/)
+___
+### Problem
+
+Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.
+
+According to the [definition of LCA on Wikipedia](https://en.wikipedia.org/wiki/Lowest_common_ancestor): “The lowest common ancestor is defined between two nodes `p` and `q` as the lowest node in `T` that has both `p` and `q` as descendants (where we allow **a node to be a descendant of itself**).”
+
+#### Examples
+
+**Example 1:**
+
+![](https://assets.leetcode.com/uploads/2018/12/14/binarysearchtree_improved.png)
+
+**Input:** root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 8
+**Output:** 6
+**Explanation:** The LCA of nodes 2 and 8 is 6.
+
+**Example 2:**
+
+![](https://assets.leetcode.com/uploads/2018/12/14/binarysearchtree_improved.png)
+
+**Input:** root = [6,2,8,0,4,7,9,null,null,3,5], p = 2, q = 4
+**Output:** 2
+**Explanation:** The LCA of nodes 2 and 4 is 2, since a node can be a descendant of itself according to the LCA definition.
+
+**Example 3:**
+
+**Input:** root = [2,1], p = 2, q = 1
+**Output:** 2
+
+#### Constraints
+
+-   The number of nodes in the tree is in the range `[2, 105]`.
+-   `-109 <= Node.val <= 109`
+-   All `Node.val` are **unique**.
+-   `p != q`
+-   `p` and `q` will exist in the BST.
+
+### Thoughts
+
+> [!summary]
+> This is a #binary_search 
+
+Because of the features of BST, the maximum val of a left sub-tree is smaller than node, so the valid LCA must meet this:
+```
+root->val >= small && root->val <= big
+```
+otherwise, search the left or right subtree.
+
+### Solution
+
+```cpp
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+ * };
+ */
+
+class Solution {
+public:
+  TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
+    // DFS-like recursion
+    // Base cases
+    int big = max(q->val, p->val);
+    int small = min(q->val, p->val);
+    
+    if (root->val >= small && root->val <= big) {
+      return root;
+    } else if (root->val > big) {
+      return lowestCommonAncestor(root->left, p, q);
+    } else {
+      return lowestCommonAncestor(root->right, p, q);
+    }
+  }
+};
+```
\ No newline at end of file
diff --git a/OJ notes/pages/Leetcode Validate-Binary-Search-Tree.md b/OJ notes/pages/Leetcode Validate-Binary-Search-Tree.md
index d085390..d331b79 100644
--- a/OJ notes/pages/Leetcode Validate-Binary-Search-Tree.md	
+++ b/OJ notes/pages/Leetcode Validate-Binary-Search-Tree.md	
@@ -15,10 +15,12 @@
 
 ##### Related topics:
 ```expander
-tag:#DFS_inorder
+tag:#DFS_inorder OR tag:#binary_search_tree
 ```
  
 - [[Leetcode Binary-Tree-Inorder-Traversal]]
+- [[Leetcode Lowest-Common-Ancestor-Of-a-Binary-Search-Tree]]
+- [[Leetcode Two-Sum-IV-Input-Is-a-BST]]
  
 
 ##### Links: