# LeetCode 98: Validate Binary Search Tree — Step-by-Step Visual Trace

**Medium** — Binary Tree | Depth-First Search | Recursion | Tree Traversal

## The Problem

Given the root of a binary tree, determine if it is a valid binary search tree where all left descendants are less than the node and all right descendants are greater than the node.

## Approach

This solution uses in-order traversal of the binary tree, which visits nodes in ascending order for a valid BST. During traversal, it maintains the previous node's value and ensures each current node is strictly greater than the previous one.

**Time:** O(n) · **Space:** O(h)

## Code

```python
class Solution:
    def isValidBST(self, root: TreeNode) -> bool:
        def inorder_traversal(node, prev):
            if not node:
                return True

            if not inorder_traversal(node.left, prev):
                return False

            if prev[0] is not None and node.val <= prev[0]:
                return False
            prev[0] = node.val

            return inorder_traversal(node.right, prev)

        prev = [None]
        return inorder_traversal(root, prev)
```

## Watch It Run

> **[Open interactive visualization](https://tracelit.dev/app?trace=0098_validate-binary-search-tree)**

> **Try it yourself:** Open [TraceLit](https://tracelit.dev/app?trace=0098_validate-binary-search-tree) and step through every line.

---

*Built with [TraceLit](https://tracelit.dev) — the visual algorithm tracer for LeetCode practice.*