中序遍历搞一波，然后看一下是不是递增的就可以了

/*** Definition for a binary tree node.* public class TreeNode {*     int val;*     TreeNode left;*     TreeNode right;*     TreeNode() {}*     TreeNode(int val) { this.val = val; }*     TreeNode(int val, TreeNode left, TreeNode right) {*         this.val = val;*         this.left = left;*         this.right = right;*     }* }*/
class Solution {long min = Long.MIN_VALUE;public boolean isValidBST(TreeNode root) {if(root == null)return true;boolean left = isValidBST(root.left);if(min >= root.val)  return false;min = root.val;boolean right = isValidBST(root.right);return left && right;}
}