From ea9341d10521ea22b3a675165b9f2465df9b4800 Mon Sep 17 00:00:00 2001
From: Pratul Trivedi <pratultrivedi@Pratuls-MacBook-Pro.local>
Date: Thu, 26 Feb 2026 18:52:28 -0800
Subject: [PATCH] Solved Trees-1

---
 TreeFromPreorderInorder.py | 66 ++++++++++++++++++++++++++++++++++++++
 ValidateBst.py             | 63 ++++++++++++++++++++++++++++++++++++
 2 files changed, 129 insertions(+)
 create mode 100644 TreeFromPreorderInorder.py
 create mode 100644 ValidateBst.py

diff --git a/TreeFromPreorderInorder.py b/TreeFromPreorderInorder.py
new file mode 100644
index 00000000..7eee09ca
--- /dev/null
+++ b/TreeFromPreorderInorder.py
@@ -0,0 +1,66 @@
+# 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]
+        """
+        if not preorder or not inorder:
+            return None
+        """
+        rootVal = preorder[0]
+        rootIdx = -1
+        for i in range(len(inorder)):
+            if inorder[i] == rootVal:
+                rootIdx = i
+                break
+    
+        root = TreeNode(rootVal)
+        leftPre = preorder[1:rootIdx+1]
+        rightPre = preorder[rootIdx+1:]
+        leftIno = inorder[0:rootIdx]
+        rightIno = inorder[rootIdx+1:]
+
+        root.left = self.buildTree(leftPre, leftIno)
+        root.right = self.buildTree(rightPre, rightIno)
+
+        return root
+        """
+
+        self.idxMap = dict()
+        for i in range(len(inorder)):
+            self.idxMap[inorder[i]] = i
+
+        self.treeRootIdx = 0
+
+        def helper(preorder, start, end):
+            if start > end:
+                #Breached. The current Node is NULL.
+                return None
+
+            #First things first, let us get the root node.
+            rootVal = preorder[self.treeRootIdx]
+            rootIdx = self.idxMap[rootVal]
+            self.treeRootIdx += 1
+            root = TreeNode(rootVal)
+
+
+            #Let us build the left and right child recursively.
+            #for left child, start remains the same, but end is the rootIdx - 1
+            root.left = helper(preorder, start, rootIdx - 1)
+
+            #for right child, start would be rootIdx+1, end remains the same.
+            root.right = helper(preorder, rootIdx + 1, end)
+
+            return root
+
+        return helper(preorder, 0, len(inorder)-1)
+
+#TC: O(n)
+#SC: O(1)
\ No newline at end of file
diff --git a/ValidateBst.py b/ValidateBst.py
new file mode 100644
index 00000000..2c2191a8
--- /dev/null
+++ b/ValidateBst.py
@@ -0,0 +1,63 @@
+# 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
+        """
+        if not root:
+            return True
+        """
+        self.breachFound=True
+        self.prev = None
+        def helper(root):
+            if not root:
+                return
+
+            helper(root.left) 
+            #Check with the previous value. Why previous?
+            # Inorder traversal of a BST will always give us
+            # the sorted value.
+            if (self.prev != None and self.prev.val >= root.val):
+                #breach
+                self.breachFound = False
+                return
+            
+            #This node looks good. Update prev and explore right subtree.
+            self.prev = root
+            helper(root.right)
+
+            return
+        helper(root)
+        return self.breachFound
+        """
+
+        """
+        TC : O(n) <--- all the nodes are visited once at max.
+        SC : O(h) <--- height of the tree. Worst case, it will be O(n)
+                       in case of a skewed tree.
+        """
+        def helper(root, minVal, maxVal):
+            if not root:
+                return True
+
+            if (root.val <= minVal or root.val >= maxVal):
+                return False
+            # If we are going left in a BST, all the numbers on the left subtree
+            # should be lesser that the currentValue. There is no bound of minimum
+            # as we don;t care about it.
+            left = helper(root.left, minVal, root.val)
+            # If we are going right in a BST, all the numbers on the right subtree
+            # should be greater that the currentValue. There is no bound of maximum
+            # as we don;t care about it.
+            right = helper(root.right, root.val, maxVal)
+
+            #Return True if no breaches found.
+            return left and right
+
+        return helper(root, float('-inf'), float('inf'))
\ No newline at end of file