add solution to 538

Author: He Hao

leetcode_python_solutions/538.py

@@ -0,0 +1,22 @@
+# 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 convertBST(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
+        if not root:
+            return None
+        self.sum = 0
+        root = self.convert_to_bst(root)
+        return root
+
+    def convert_to_bst(self, root):
+        if not root:
+            return None
+        self.convert_to_bst(root.right)
+        self.sum += root.val
+        root.val = self.sum
+        self.convert_to_bst(root.left)
+        return root