解题思路：

class Solution {
    public boolean isValidBST(TreeNode root) {
        return recur(root,Long.MIN_VALUE,Long.MAX_VALUE);
    }
    public boolean recur(TreeNode root,long lower,long upper){
        if(root==null) return true;
        if(root.val<=lower||root.val>=upper) return false;
        return recur(root.left,lower,root.val)&&recur(root.right,root.val,upper);
    }
}