Validate Binary Search Tree

public class Solution {
    public boolean isValidBST(TreeNode root) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(root == null)
            return true;
        if(root.left != null)
        {
            TreeNode left = root.left;
            while(left.right != null)
                left = left.right;
            if(root.val <= left.val)
                return false; 
        }
        if(root.right != null)
        {
            TreeNode right = root.right;
            while(right.left != null)
                right = right.left;
            if(root.val >= right.val)
                return false;
        }
        return isValidBST(root.left) && isValidBST(root.right);
    }
}