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-# Algorithms 2019-2020 Python LEETCODE
-
-Check branches for information.
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+# Binary tree
+
++ [Validate Binary Search Tree (pythonic way)](#validate-binary-search-tree-pythonic-way)
++ [Validate Binary Search Tree (recursive)](#validate-binary-search-tree-recursive)
+
+## Validate Binary Search Tree (pythonic way)
+
+https://leetcode.com/problems/validate-binary-search-tree/
+
+```python
+# Definition for a binary tree node.
+# class TreeNode(object):
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+
+
+class Solution(object):
+    def isValidBST(self, _root, first=True):
+        """
+        :type _root: TreeNode
+        :type first: bool
+        :rtype: bool
+        """
+        root = _root
+        if not root:
+            return first or []
+        result = self.isValidBST(root.left, False) + [root.val] + self.isValidBST(root.right, False)
+        return all([right > left for right, left in zip(result[1:], result)]) if first else result
+
+```
+
+## Validate Binary Search Tree (recursive)
+
+https://leetcode.com/problems/validate-binary-search-tree/
+
+```python
+# Definition for a binary tree root.
+# class TreeNode(object):
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution(object):
+    def check(self, root, left=None, right=None):
+        """
+        :type root: TreeNode
+        :type left: TreeNode
+        :type right: TreeNode
+        :rtype: bool
+        """
+        if not root:
+            return True
+        if left is not None and root.val <= left:
+            return False
+        if right is not None and root.val >= right:
+            return False
+        return self.check(root.left, left, root.val) and self.check(root.right, root.val, right)
+
+    def isValidBST(self, _root):
+        """
+        :type _root: TreeNode
+        :rtype: bool
+        """
+        root = _root
+        return self.check(root)
+
+```
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