diff --git a/Problem_1.py b/Problem_1.py
new file mode 100644
index 00000000..8d86e562
--- /dev/null
+++ b/Problem_1.py
@@ -0,0 +1,26 @@
+# Definition for a binary tree node.
+import math
+
+
+class TreeNode:
+    def __init__(self, val=0, left=None, right=None):
+        self.val = val
+        self.left = left
+        self.right = right
+
+class Solution:
+    def isValidBST(self, root):
+        self.result = True
+        self.helper(root, -math.inf, math.inf)
+        return self.result
+
+    def helper(self, root, min_range, max_range):
+        if not root:
+            return
+        if not min_range < root.val < max_range:
+            self.result = False
+            return
+        self.helper(root.left, min_range, root.val)
+        self.helper(root.right, root.val, max_range)
+
+
diff --git a/Problem_2.py b/Problem_2.py
new file mode 100644
index 00000000..1c32f491
--- /dev/null
+++ b/Problem_2.py
@@ -0,0 +1,24 @@
+# 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, inorder):
+        self.index = 0
+        self.inorder_map = {}
+        for i in range(len(inorder)):
+            ele = inorder[i]
+            self.inorder_map[ele] = i
+        return self.helper(preorder, 0, len(inorder) - 1)
+
+    def helper(self, preorder, start, end):
+        if start > end:
+            return None
+        root = TreeNode(preorder[self.index])
+        idx = self.inorder_map[root.val]
+        self.index += 1
+        root.left = self.helper(preorder, start, idx - 1)
+        root.right = self.helper(preorder, idx + 1, end)
+        return root