update readme

Author: tech-cow

README.md

@@ -72,5 +72,5 @@ Success is like pregnancy, Everybody congratulates you but nobody knows how many
 |100|[Same Tree](https://leetcode.com/problems/same-tree/)| [Python [Yu]](./tree/Yu/100_isSameTree.py) <br> [Python [Sheng]](./tree/YeSheng/100.Same_Tree.py) | _O(N)_| _O(1)_  | Easy ||[EAFP vs. LBYL](https://github.com/ictar/python-doc/blob/master/Others/%E5%AE%9E%E7%94%A8Python%EF%BC%9AEAFP%20VS.%20LBYL.md)|
 |101|[Symmetric Tree](https://leetcode.com/problems/symmetric-tree/#/description)| [Python [Yu]](./tree/Yu/101_symmetricTree.py) <br> [Python [Sheng]](./tree/YeSheng/101.Symmetric_Tree.py) | _O(N)_| _O(1)_  | Easy ||[Editorial Solution](https://leetcode.com/articles/symmetric-tree/)|
 |111|[Minimum Depth of Binary Tree](https://leetcode.com/problems/minimum-depth-of-binary-tree/#/description)| [Python [Yu]](https://github.com/yuzhoujr/LeetCode/blob/master/tree/Yu/111_minDepth.py) <br> [Python [Sheng]](https://github.com/yuzhoujr/LeetCode/blob/master/tree/YeSheng/111.Minimum_Depth_of_Binary_Tree.py) | _O(N)_| _O(1)_  | Easy |||
-|104|[Maximum Depth of Binary Tree](https://leetcode.com/problems/maximum-depth-of-binary-tree/)| [Python](./tree/Yu/104_maxDepth.py) | _O(1)_| _O(n)_  | Easy | ||
-|235|[Lowest Common Ancestor of a Binary Search Tree](https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/#/description)| [Python](./tree/Yu/235_lca_bst.py) | _O(1)_| _O(n)_  | Easy | ||
+|104|[Maximum Depth of Binary Tree](https://leetcode.com/problems/maximum-depth-of-binary-tree/)| [Python](./tree/Yu/104_maxDepth.py) | _O(N)_| _O(1)_  | Easy | ||
+|235|[Lowest Common Ancestor of a Binary Search Tree](https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/#/description)| [Python](./tree/Yu/235_lca_bst.py) | _O(N)_| _O(1)_  | Easy | ||

tree/Yu/235_lca_bst.py

@@ -5,7 +5,28 @@
 
 # ****************
 # Descrption:
-# Given a binary search tree (BST), find the lowest common ancestor (LCA) of two given nodes in the BST.
+
+# Time:  O(n)
+# Space: O(1)
+#
+# 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: “The lowest common ancestor
+# is defined between two nodes v and w as the lowest node in T that has both v
+# and w as descendants (where we allow a node to be a descendant of itself).”
+#
+#         _______6______
+#       /              \
+#     ___2__          ___8__
+#   /      \        /      \
+#   0      _4       7       9
+#          /  \
+#          3   5
+# For example, the lowest common ancestor (LCA) of nodes 2 and 8 is 6.
+# Another example is LCA of nodes 2 and 4 is 2, since a node can be a
+# descendant of itself according to the LCA definition.
+
 # ****************
 
 # 思路：