# Question

Formatted question description: https://leetcode.ca/all/98.html

98	Validate Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:
The left subtree of a node contains only nodes with keys less than the node's key.
The right subtree of a node contains only nodes with keys greater than the node's key.
Both the left and right subtrees must also be binary search trees.

@tag-tree


# Algorithm

Make use of the nature of BST, bring the maximum and minimum values of the system during initialization, and replace them with their own node values in the recursive process.

The use of long instead of int is to include the boundary conditions of int.

For below BFS solution, we can also define a new data structure MyNode


class MyNode {

TreeNode node;
int upper;
int lower;
}


Then initiate as

new MyNode(node, Integer.Max, Integer.Min)


And enqueue as

queue.offer(new MyNode(node.left, lower, currentNode.val))
queue.offer(new MyNode(node.right, currentNode.val, upper))


### Note: remember below in invalid, 7 should go right of 5

   5
/ \
1   8
\   \
7   10


Java