working solution

problem1.py

@@ -0,0 +1,26 @@
+# problem 1 - https://leetcode.com/problems/validate-binary-search-tree/
+
+# 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 __init__(self):
+        self.flag = True
+
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+        self.helper(root,float(-inf),float(inf))
+        return self.flag
+
+    def helper(self,node,min_val,max_val):
+        if not node:
+            return
+        if node.val >= max_val or node.val <= min_val:
+            self.flag = False 
+        self.helper(node.left,min_val,node.val)
+        self.helper(node.right,node.val,max_val)
+
+

problem2.py

@@ -0,0 +1,23 @@
+# problem 2 
+
+# 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 buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
+        if len(preorder) == 0:
+            return None
+        root_val = preorder[0]
+        root_val_idx = inorder.index(root_val)
+
+        left_inorder = inorder[:root_val_idx]
+        right_inorder = inorder[root_val_idx+1:]
+        left_preorder = preorder[1:len(left_inorder)+1]
+        right_preorder = preorder[len(left_inorder)+1:]
+        node = TreeNode(root_val)
+        node.left = self.buildTree(left_preorder,left_inorder)
+        node.right = self.buildTree(right_preorder,right_inorder)
+        return node