98 validate binary search tree

https://leetcode.com/problems/validate-binary-search-tree/

/**
 * 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 {
    TreeNode cur;
    public boolean isValidBST(TreeNode root) {
        return validate(root);
    }
    private boolean validate(TreeNode root) {
        if (root == null) return true;
        if (!validate(root.left)) return false;
        if (cur != null && cur.val >= root.val) return false;
        cur = root;
        if (!validate(root.right)) return false;
        return true;
    }
}

有一阵子没有做过这个题了诶 利用了中序遍历的有序性