Is this valid? It sure seems so, the first criteria that comes to mind being that 3 is less than 5, and therefore that node is to the left. On the other hand, 9 is greater than 5, and thus is to the right. Alright, this seems like a no brainer, let's just check that every left child is smaller than the root, and every right child is greater than the local root.

Brute force solution?

Here's a possible early solution:

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function isValidBST(root) {

  if (root.left < root.val && root.right > root.val) {

    return true;

  }

}