CTCI 4.5

/* The inorder travel of a BST is strictly increasing. We track the pre node of each 
node during the recursive process, and compare the two node‘s value to determine whether 
the tree is BST*/

public class IsBinarySearchTree {
    boolean stat = true;
    public boolean isValidBST(TreeNode root) {
        inorder(root, null);
        return stat;
    }
    
    private TreeNode inorder(TreeNode node, TreeNode pre) {
        if(node == null) return pre;
        else {
            TreeNode temp = inorder(node.left, pre);
            if(temp != null && node != temp && node.val <= temp.val) stat = false;
            temp = inorder(node.right, node);
            return temp;
        }
    }
}