98. Validate Binary Search Tree

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Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

  • The left subtree of a node contains only nodes with keys less than the node's key.
  • The right subtree of a node contains only nodes with keys greater than the node's key.
  • Both the left and right subtrees must also be binary search trees.

Example 1:

    2
   / \
  1   3
Binary tree [2,1,3], return true.

Example 2:

    1
   / \
  2   3
Binary tree [1,2,3], return false.


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