diff --git a/Problem1.py b/Problem1.py
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+++ b/Problem1.py
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+#Problem 98. VALIDATE BINARY SEARCH TREE
+# TIME COMPELXITY: O (N) where N is the number of nodes
+# SPACE COMPLEXITY: O(H) where H is the height of the tree
+
+
+# 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):
+        """
+        :type root: Optional[TreeNode]
+        :rtype: bool
+        """
+        
+        return self.helper(root,None,None)
+
+    def helper(self,root,minValue,maxValue):
+        if root is None:
+            return True
+            
+        if minValue is not None and root.val<=minValue:
+            return False
+            
+        if maxValue is not None and root.val>=maxValue:
+            return False
+
+        return self.helper(root.left,minValue,root.val) and self.helper(root.right,root.val,maxValue)
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diff --git a/Problem2.py b/Problem2.py
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+++ b/Problem2.py
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+#Problem 105. CONSTRUCT BINARY TREE FROM PREORDER AND INORDER TRAVERSAL
+# TIME COMPELXITY: O (N) where N is the number of nodes
+# SPACE COMPLEXITY: O (N) where N is the number of nodes
+
+
+# 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 buildTree(self, preorder, inorder):
+        """
+        :type preorder: List[int]
+        :type inorder: List[int]
+        :rtype: Optional[TreeNode]
+        """
+        self.index=0
+        inordermap={val:i for i,val in enumerate(inorder)}
+        return self.helper(preorder,0,len(inorder)-1,inordermap)
+
+    def helper(self,preorder,start,end,inordermap):
+        if start>end:
+            return None
+        root_val=preorder[self.index]
+        self.index+=1
+        root=TreeNode(root_val)
+
+        root_idx=inordermap[root_val]
+        root.left=self.helper(preorder,start,root_idx-1,inordermap)
+        root.right=self.helper(preorder,root_idx+1,end,inordermap)
+
+        return root
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