best-buy-sell-price.js

best-buy-sell-price.js

function bestBuyAndSellPrices(stockPrices) {
  let buyDay = null;
  let sellDay = null;
  let buyPrice = typeof stockPrices[0] !== 'number' ? null : stockPrices[0];
  let sellPrice = buyPrice;

  stockPrices.forEach((currentBuyPrice, i) => {
    const prevPrice = stockPrices[i - 1];
    const nextPrice = stockPrices[i + 1];
    const isBuyPrice = ((currentBuyPrice - prevPrice) < 0 && (nextPrice - currentBuyPrice) >= 0);

    // only analyze prices if we've bottomed out on a dip
    if (!isBuyPrice) return;

    let n = i + 1;

    // find the best succeeding price to sell at
    while (n < stockPrices.length) {
      const currentSellPrice = stockPrices[n];
      const profit = currentSellPrice - currentBuyPrice;

      if (profit > (sellPrice - buyPrice)) {
        buyDay = i + 1;
        buyPrice = currentBuyPrice;
        sellDay = n + 1;
        sellPrice = currentSellPrice;
      }

      n++;
    }
  });

  return {
    buyDay,
    buyPrice,
    sellDay,
    sellPrice,
    profit: sellPrice - buyPrice,
  };
}

console.log(bestBuyAndSellPrices([12, 17, 1, 12, 15, 7, 14, 16, 8, 6]));
console.log(bestBuyAndSellPrices([17, 19, 18, 14, 20, 21, 14, 19, 18, 6]));
console.log(bestBuyAndSellPrices([20, 3, 18, 6, 11, 15, 1, 12, 9, 3, 1, 17, 18, 20, 11]));
console.log(bestBuyAndSellPrices([4]));
console.log(bestBuyAndSellPrices([]));