Tree DFS

function isValidBST(root: TreeNode | null): boolean {
  const check = (node: TreeNode | null, low: number, high: number): boolean => {
    if (!node) return true;
    if (node.val <= low || node.val >= high) return false;
    return check(node.left, low, node.val) && check(node.right, node.val, high);
  };
  return check(root, -Infinity, Infinity);
}

def valid_bst?(node, low = -Float::INFINITY, high = Float::INFINITY)
  return true unless node
  return false if node.val <= low || node.val >= high

  valid_bst?(node.left, low, node.val) && valid_bst?(node.right, node.val, high)
end