98. 验证二叉搜索树

中序遍历 (边遍历边验证顺序性)

   private TreeNode prev = null;
    private boolean isBST = true;
    public boolean isValidBST(TreeNode root) {

        inorder(root);

        return isBST;
    }

    private void inorder(TreeNode node) {
        if (node == null) return;

        inorder(node.left);
        if (prev != null && node.val <= prev.val) {
            // 不符合 BST 性质
            isBST = false;
            return;
        }
        prev = node;
        inorder(node.right);
    }