[IMP] Trees-1

construct-binary-tree-from-preorder-and-inorder-traversal.py

@@ -0,0 +1,36 @@
+''' 
+Time Complexity : O(2) 
+Space Complexity : O(n) 
+Did this code successfully run on Leetcode : Yes
+Any problem you faced while coding this :  No
+'''
+# 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
+        rootVal = preorder[0]
+        rootIdx = -1
+        for i in range(len(inorder)):
+            if inorder[i] == rootVal:
+                rootIdx = i
+                break
+
+        root = TreeNode(rootVal)
+        inleft = inorder[:rootIdx]
+        inRight = inorder[rootIdx+1:]
+
+        preleft = preorder[1:len(inleft)+1]
+        preRight = preorder[len(inleft)+1:]
+
+        root.left = self.buildTree(preleft, inleft)
+        root.right = self.buildTree(preRight, inRight)
+
+        return root
+
+

validate-binary-search-tree.py

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+''' 
+Time Complexity : O(2) 
+Space Complexity : O(n) 
+Did this code successfully run on Leetcode : Yes
+Any problem you faced while coding this :  No
+'''
+
+# 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
+import copy
+class Solution:
+    def __init__(self):
+        self.flag = True
+        self.prev = None
+
+    def helper(self, root):
+        if root == None:
+            return
+
+        self.helper(root.left)
+
+        # write condition to make the flag false
+        if self.prev != None and self.prev.val >= root.val:
+            self.flag = False
+        self.prev = root
+
+        self.helper(root.right)
+
+    def isValidBST(self, root: Optional[TreeNode]) -> bool:
+
+        if root == None:
+            return True
+
+        self.helper(root)
+
+        return self.flag