98. Validate Binary Search Tree

 1 class Solution {
 2     public boolean isValidBST(TreeNode root) {
 3         return helper(root,(long)Integer.MAX_VALUE + 1,(long)Integer.MIN_VALUE - 1); //万一有node就是maxvalue 所以要+1 min同理
 4         
 5     }
 6     
 7     public boolean helper(TreeNode root,long max,long min) {
 8         if(root == null) return true;
 9         return ((root.val > min) && (root.val < max) && (helper(root.left,Math.min(max,root.val),min)) && (helper(root.right,max,Math.max(min,root.val))));
10     }
11 }