[LC 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) {
        return isValidBST(root,null,null);

    }
    public boolean isValidBST(TreeNode root,TreeNode min,TreeNode max){
        if(root == null){
            return true;
        }
        if(min != null && root.val<=min.val)
            return false;
        if(max != null && root.val>=max.val)
            return false;
        return isValidBST(root.left,min,root)&&isValidBST(root.right,root,max);
    }
}