力扣 递归 98. 验证二叉搜索树

/**

 * 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 {

    public boolean isValidBST(TreeNode root) {

        if(root == null) {

            return true;

        }

        return isValid(root, Long.MIN_VALUE, Long.MAX_VALUE);

    }

    public boolean isValid(TreeNode node, long lower, long larger) {

        if (node == null) {

            return true;

        }

        if (node.val <= lower || node.val >= larger) {

            return false;

        }

       return isValid(node.left, lower, node.val) &&  isValid(node.right,node.val, larger);

    }

}