还是没做出来，这道题的方法是比较特别，记住吧。为什么要初始化为null,是因为不知道上下限。
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public boolean isValidBST(TreeNode root) {
if (root == null){
return true;
}
return helper(root, null, null);
}
private boolean helper(TreeNode root, Integer min, Integer max){
if (root == null){
return true;
}
if (min != null && root.val <= min || (max != null && root.val >= max)){
return false;
}
return helper(root.left, min, root.val) && helper(root.right, root.val, max);
}
}
