How should I practice Validate Binary Search Tree?

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Validate Binary Search Tree

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Problem statement

Given the root of a binary tree, return whether it is a valid binary search tree.

For every node, every value in its left subtree must be strictly smaller than the node's value, every value in its right subtree must be strictly greater, and both subtrees must satisfy the same rule. Duplicate values therefore make the tree invalid.

Function

isValidBST(root: TreeNode) → boolean

Examples

Example 1

root = [2,1,3]return = true

Both children satisfy the strict bounds imposed by the root.

Example 2

root = [5,1,4,null,null,3,6]return = false

Node 4 is in the root's right subtree but is smaller than root value 5.

Constraints

The tree contains between 1 and 10000 nodes.
-2147483648 <= Node.val <= 2147483647.