Given a binary tree, determine if it's a valid binary search tree (BST)
Assume a BST is defined as follow:
- 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 by BSTs
5
/ \
/ \
1 4
/ \
3 6
Output: False (3 is less than 5)