LeetCode Q98: 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.

Solution 1:

Inorder traverse the tree and check whether all nodes visited are sorted in an ascending order.

Solution 2:

Recursively check whether the BST properties are hold in left and right subtrees. In the meantime, we need to keep tracking the low and high bound of each side.