【面试】判断一棵二叉树是否为二叉排序树

class TreeNode {
    int key;
    TreeNode left;
    TreeNode right;
    public TreeNode(int key) {
        this.key = key;
    }
}

/**
 * Created by LEESF on 2016/9/8.
 */
class TreeNode {
    int key;
    TreeNode left;
    TreeNode right;
    public TreeNode(int key) {
        this.key = key;
    }
}
public class IsBSTree {
    static boolean flag = true;
    static int last = Integer.MIN_VALUE;
    public static boolean isBSTree(TreeNode root) {
        if (root.left != null && flag)
            isBSTree(root.left);
        if (root.key < last)
            flag = false;
        last = root.key;
        if (root.right != null && flag)
            isBSTree(root.right);
        return flag;
    }
}