/* 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;        }    }}