99_Recover_Binary_Search_Tree

Author: Partho Biswas

README.md

@@ -446,6 +446,7 @@ Check this [golden](https://tinyurl.com/ujopecz) post.
 |12| **[449. Serialize and Deserialize BST](https://tinyurl.com/rx9rn3l)** | [Python](https://tinyurl.com/wu6rdaw/449_Serialize_and_Deserialize_BST.py), [Swift](https://tinyurl.com/wuja3c4/449_Serialize_and_Deserialize_BST.swift)| [Art 1](https://tinyurl.com/t6e2b97) | Medium | --- |
 |13| **[1008. Construct Binary Search Tree from Preorder Traversal](https://tinyurl.com/y6less2m)** | [Python](https://tinyurl.com/wu6rdaw/1008_Construct_Binary_Search_Tree_from_Preorder_Traversal.py), [Swift](https://tinyurl.com/wuja3c4/1008_Construct_Binary_Search_Tree_from_Preorder_Traversal.swift)| [Art 1](https://tinyurl.com/y9mondek), [Art 2](https://tinyurl.com/y8byt732), [Art 3](https://tinyurl.com/yaq7w2jt), [Art 4](https://tinyurl.com/ybwtzpzo) | Medium | --- |
 |14| **[653. Two Sum IV - Input is a BST](https://tinyurl.com/ydzdnawg)** | [Python](https://tinyurl.com/wu6rdaw/653_Two_Sum_IV_Input_is_a_BST.py), [Swift](https://tinyurl.com/wuja3c4/653_Two_Sum_IV_Input_is_a_BST.swift)| - | Easy | --- |
+|15| **[99. Recover Binary Search Tree](https://tinyurl.com/y9zxw8aj)** | [Python](https://tinyurl.com/wu6rdaw/99_Recover_Binary_Search_Tree.py), [Swift](https://tinyurl.com/wuja3c4/99_Recover_Binary_Search_Tree.swift)| - | Hard | **Very important, check again** |
 
 </p>
 </details>

leetcode.com/python/99_Recover_Binary_Search_Tree.py

@@ -0,0 +1,31 @@
+# Definition for a binary tree node.
+# class TreeNode(object):
+#     def __init__(self, x):
+#         self.val = x
+#         self.left = None
+#         self.right = None
+
+class Solution(object):
+    def recoverTree(self, root):
+        """
+        :type root: TreeNode
+        :rtype: None Do not return anything, modify root in-place instead.
+        """
+        stack = []
+        x = y = predecessor = None # predecessor of the current node https://tinyurl.com/ycrzmfzg
+
+        while stack or root:
+            while root:
+                stack.append(root)
+                root = root.left
+            root = stack.pop()
+            if predecessor and root.val < predecessor.val:
+                y = root
+                if not x:
+                    x = predecessor
+                else:
+                    break
+            predecessor = root
+            root = root.right
+
+        x.val. y.val = y.val, x.val