Check if a binary tree is a valid BST (with recursion)?

Short answer: bool IsValidBST(TreeNode root) { return Validate(root, null, null); } Follow on: bool Validate(TreeNode node, int?

Explain a bit more

min, int? max) { if (node == null) return true; if ((min != null && node.val <= min) || (max != null && node.val >= max)) return false; return Validate(node.left, min, node.val) && Validate(node.right, node.val, max); } Explanation: Pass down min and max bounds for subtree values; node must be in (min, max) range.

Example code

bool IsValidBST(TreeNode root) { return Validate(root, null, null);
} Follow on: bool Validate(TreeNode node, int? min, int? max) { if (node == null) return true;
if ((min != null && node.val <= min) || (max != null && node.val
>= max)) return false;
return Validate(node.left, min, node.val) && Validate(node.right, node.val, max); } Explanation: Pass down min and max bounds for subtree values; node must be in (min, max) range.

Real-world example (ShopNest)

In coding rounds, state complexity aloud, write a clear ShopNest-flavored example (orders, carts), then handle edge cases (empty list, null, overflow).

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.