JSFiddle - React, Tailwind, and code Playground

JavaScript

const findWinner = (n) => {
  if (!n || n < 1) {
    return null;
  }
  let totalStones = 0;
  let player = 1; // A
  while (totalStones < n) {
    totalStones += player ? 1 : 2;
    player = !player;
  }
  return player ? 'A' : 'B';
};


/** Test cases **/
console.log(findWinner(1));      // B
console.log(findWinner(2));      // A
console.log(findWinner(3));      // A
console.log(findWinner(4));      // B
console.log(findWinner(0));      // null
console.log(findWinner(null));   // null