stone-game-ii-works.js

var stoneGameII = function(piles) {
  return stoneGameRec(piles, 1, [])
};

function stoneGameRec(piles, M, memo) {
  if (piles.length === 0) return 0
  if (piles.length === 1) return piles[0]
  if (piles.length === 2) return piles[0] + piles[1]
  
  const totalStones = piles.reduce((sum, e) => sum + e, 0)
  
  let stones = 0
  for (let i=1; i <= 2 * M; i++) {
    const result = stoneGameRec(piles.slice(i), Math.max(i, M), memo)
    
    if (totalStones - result > stones) {
      stones = totalStones - result
    }
  }

  return stones
}

stone-game-ii.js

var stoneGameII = function(piles) {
  return stoneGameRec(piles, 0, 1, [])
};

function stoneGameRec(piles, index, M, memo) {
  if (index === piles.length) return 0
  if (index === piles.length - 1) return piles[index]
  if (index === piles.length - 2) return piles[index] + piles[index+1]
  
  if (memo[index]) return memo[index]
  
  let totalStones = 0
  for (let i=index; i < piles.length; i++) {
    totalStones += piles[i]
  }
  
  let stones = 0
  for (let i=index; i <= index + 2 * M; i++) {
    const result = stoneGameRec(piles, i+1, Math.max(i, M), memo)
    
    if (totalStones - result > stones) {
      stones = totalStones - result
    }
  }
  
  memo[index] = stones
  return stones
}