ValidateBST.java

/**
 * 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 Validate(root,null,null);
    }
    public boolean Validate(TreeNode root,Integer max, Integer min)
    {
        if(root == null)
            return true;
        else if(max!=null && root.val>=max || min != null && root.val<=min)
            return false;
        else
            return Validate(root.left,root.val,min) && Validate(root.right ,max, root.val);
    }
}