c# solution, O(1) space, recursion, 7 lines

  • 0
    public class Solution {
       int? prev = null;
            public bool IsValidBST(TreeNode node)
                 if (node == null) return true;
                bool isLeft = true;
                if (node.left != null) isLeft = IsValidBST(node.left);
                if (!isLeft || (node.val <= prev)) return false;
                prev = node.val;
                return IsValidBST(node.right);

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