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Validate Binary Search Tree

MediumTrees

Given the root of a binary tree (as a level-order array), determine if it is a valid binary search tree (BST).

A valid BST has:

  • Left subtree contains only nodes with keys less than the node's key
  • Right subtree contains only nodes with keys greater than the node's key
  • Both subtrees must also be valid BSTs

Example:

Input:
2,1,3
Output:
True
Reasoning:
  • The input array 2,1,3 represents a binary tree with root node 2, left child 1, and right child 3.
  • We check the left subtree: since 1 is less than 2, it satisfies the BST condition.
  • We check the right subtree: since 3 is greater than 2, it satisfies the BST condition.
  • Both subtrees are valid BSTs (as they are single nodes with no children), so the entire tree is a valid binary search tree, resulting in an output of True.

Constraints:

  • 1 <= number of nodes <= 10^4
  • -2^31 <= Node.val <= 2^31 - 1
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