Using Enumerable Set From OZ

Test.sol

// SPDX-License-Identifier: agpl-3.0
pragma solidity ^0.6.12;

import "./Enumerable.sol";

contract Test{

  using EnumerableSet for EnumerableSet.UintSet;
  
  mapping (address => EnumerableSet.UintSet) private ownedCards;
  
  function addCardToUser(address user, uint cardId) external{
      ownedCards[user].add(cardId);
  }
  
  function removeCardFromUser(address user, uint cardId) external{
      ownedCards[user].remove(cardId);
  }
  
  function isCardOwned(address user, uint cardId) external view returns(bool){
      return ownedCards[user].contains(cardId);
  }
  
  function getAllCards(address user) external view returns(uint[] memory){
      uint totalCards = ownedCards[user].length();
      
      uint[] memory cards = new uint[](totalCards);
      
      for(uint i=0; i<totalCards; i++){
         cards[i] =  ownedCards[user].at(i);
      }
      
      return cards;
  }
    
}