Using points within a subset to find max/min and profits from sale of a stock

find-max-stock-buy-self-profit-es6.js

/**
  // using built-in functions
  */
function findMaxStockProfit(arr1) {
  if (arr1.length < 2) {
    return 0;
  }
  let arr1Min = Math.min(...arr1);
  let subset = arr1.slice(arr1.indexOf(arr1Min));

  // these two calls to math.min, math.max on subset
  // hide the loop that is being done.
  let numberBuy 	= Math.max(...subset);
  let numberSell 	= Math.min(...subset);

  let numberProfit = numberBuy - numberSell;

  return numberProfit;
}

console.log(findMaxStockProfit([7,1,5,3,6,4]));

find-max-stock-buy-sell-profits.js

/**
  // no built-in functions.
  // 1. We have to find the min. number in the list
  // 2. We have to find the max. number in the last, after the position of min.
  // 3. no same day. and buy before you sell.
  */
function findMaxStockProfit(arr) {
  // need a larger sample size. 1 day wont work.
  if (arr.length < 2) {
    return 0;
  }   
  // set current min. and current max.
  let profit    = 0;
  let minPos		= 0;
  let maxPos		= 0;
  let maxSoFar 	= arr[0];
  let minSoFar 	= arr[0];
  for(let i=0, x=1; i < arr.length; i++, x=i+1) {
    // capture the minimum amount.
    let currentPrice = arr[i];
    let nextdayPrice = arr[x];
    // subproblem1: is my current price cheaper than tomorrow
		// is my currentPrice greater than my cheapest price? 
		// and, if so, is my nextdayPrice any cheaper?
		// (if the min price is greater than todays price, todays price is cheaper)
    if (minSoFar > nextdayPrice) {
      // yes
      // update the least price to reflect this one
      minSoFar = nextdayPrice;
      minPos   = x;
    }
    // means we found our min. after we found our max.
    if (minPos > maxPos) {
      maxSoFar 	= currentPrice;
      maxPos 		= i;
    }
    // if our max is less than current, we just found our new max.
    if (currentPrice > maxSoFar) {
      maxSoFar = currentPrice;
    }
  }
  // Profit = How much it cost me - How much I sold it for. 
  // (This should be positive, not always though)
  profit = ( maxSoFar - minSoFar );
  // no negative profits today.
	return (profit > 0)
          ? profit
          : 0
          ;
}
console.log(findMaxStockProfit([7,1,5,3,6,4]));