Validate Binary Search Tree (LC 98)

A tree that breaks every test you write

A Binary Search Tree has one rule: every node's left subtree is STRICTLY LESS than the node, and every node's right subtree is STRICTLY GREATER. Given a tree, return true if it satisfies the rule everywhere — false otherwise.

Here is the tree. Five values, four edges, two levels of children. Look at every parent and its children below — does anything look wrong?

The BST rule sounds simple — left smaller, right bigger. But there is a subtlety in that definition that trips most implementations. Look carefully at the tree before you commit.