problem: 0230 kth smallest element in a BST

Author: cychiu8

0230 Kth Smallest Element in a BST/0230 Kth Smallest Element in a BST.md

@@ -0,0 +1,47 @@
+# 0230. Kth Smallest Element in a BST
+
+* Difficulty: medium
+* Link: https://leetcode.com/problems/kth-smallest-element-in-a-bst/
+* Topics: DFS-BFS
+
+
+# Clarification
+
+1. Check the inputs and outputs
+
+# Solution (DFS)
+
+### Thought Process
+
+- ****Recursive Inorder Traversal****
+    
+    ![Untitled](./Untitled.png)
+    
+- Implement
+    
+    ```python
+    # Definition for a binary tree node.
+    # class TreeNode:
+    #     def __init__(self, val=0, left=None, right=None):
+    #         self.val = val
+    #         self.left = left
+    #         self.right = right
+    class Solution:
+        def kthSmallest(self, root: Optional[TreeNode], k: int) -> int:
+            def inorder(root):
+                if not root:
+                    return []
+                return inorder(root.left) + [root.val] + inorder(root.right)
+            
+            return inorder(root)[k-1]
+    ```
+    
+
+### Complexity
+
+- Time complexity: *O*(*N*) to build a traversal.
+- Space complexity: *O*(*N*) to keep an inorder traversal.
+
+# Note
+
+- BST 的中序遍歷結果是有序的(升冪)