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