Find the winner - CollegeStash

Lets say you want to find the winner of a game where the participants pick up stones (n). If ‘A’ and ‘B’ are the 2 players, then the rules of the game are as follows:

Player ‘A’ always starts first.
Player ‘A’ always picks 1 stone.
Player ‘B’ always picks 2 stones.
Player ‘A’ and Player ‘B’ take alternate turns.
The player to pick up the last stone is the loser.

Write a method that returns the winner. There can be 3 possible outcomes “A | B | null”.

/** 
 * @param {number} n
 * @return {'A' | 'B' | null}
 */
function findWinner(n) {
  // A : true
  // B : false
  if (!n || n <= 0)
    return null;
  return whoWins(n - 1, true) ? 'A' : 'B';
}

function whoWins(n, player) {
  if (n <= 0)
    return !player;
  return whoWins(n - (player ? 2 : 1), !player);
}

/** 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

Demo

A shorter version of the above code in ES6.

const findWinner = (n) => (!n || n <= 0) ? null : whoWins(n - 1, true) ? 'A' : 'B';
const whoWins = (n, player) => (n <= 0) ? !player : whoWins(n - (player ? 2 : 1), !player);

/** 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

Demo

Iterative Solution

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';
};

Demo

Using for loop

const findWinner = (n) => {
  if (!n || n < 1) {
    return null;
  }
  let player = 1; // A
  for (let totalStones = 0; 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

Demo

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