I have received RSU while I was working abroad and not US taxpayer. I exercised them last year and my total cot basis is 0. Is that correct. I

Perhaps.  In the US, when RSUs vest that triggers taxable wages that you pay income tax on.  If the same happened to you when your RSUs vested, then your basis in the stock equals the amount of wage income your reported at that time for whatever country you paid taxes.  If you didn't pay taxes when the RSUs vested or otherwise didn't pay anything for them, then your basis is zero