答案

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isValidBST(TreeNode root,int min,int max)
    {
        if(root==null)
        {
            return true;
        }
        if(root.val<=min||root.val>=max)
        {
            return false;
        }
        return isValidBST(root.left,min,Math.min(root.val,max))&&isValidBST(root.right,Math.max(min,root.val),max);
    }
    public boolean isValidBST(TreeNode root) {
        if(root==null)
        {
            return true;
        }
        return isValidBST(root.left,Integer.MIN_VALUE,root.val)&&isValidBST(root.right,root.val,Integer.MAX_VALUE);
    }
}