How do you validate if a binary tree is a valid BST?

Use recursion with min/max range validation. Start with range (-infinity, infinity). For each node, check if its value is within the valid range. Recursively validate left subtree with (min, node.value) and right subtree with (node.value, max). If any node violates its range, return false. Alternative: in-order traversal should produce strictly increasing sequence. Both approaches are O(n) time and O(h) space.