Stock maximum profit

You will be given a list of stock prices for a given day and your goal is to return the maximum profit that could have been made by buying a stock at the given price and then selling the stock later on.

For example if the input is:

[45, 24, 35, 31, 40, 38, 11] 

then your program should return 16 because if you bought the stock at $24 and sold it at $40, a profit of $16 was made and this is the largest profit that could be made. If no profit could have been made, return -1.

We’ll solve the challenge the following way:

1. Iterate through each number in the list.
2. At the ith index, get the i+1 index price and check if it is larger than the ith index price.
3. If so, set buy_price = i and sell_price = i+1. Then calculate the profit: sell_price - buy_price.
4. If a stock price is found that is cheaper than the current buy_price, set this to be the new buying price and continue from step 2.
5. Otherwise, continue changing only the sell_price and keep buy_price set.

This algorithm runs in linear time, making only one pass through the array, so the running time in the worst case is O(n).

function StockPicker(arr) { 
  
  var max_profit = -1;
  var buy_price = 0;
  var sell_price = 0;
  
  // this allows our loop to keep iterating the buying
  // price until a cheap stock price is found
  var change_buy_index = true;
  
  // loop through list of stock prices once
  for (var i = 0; i < arr.length-1; i++) {
    
    // selling price is the next element in list
    sell_price = arr[i+1]; 
    
    // if we have not found a suitable cheap buying price yet
    // we set the buying price equal to the current element
    if (change_buy_index) { buy_price = arr[i]; }
    
    // if the selling price is less than the buying price
    // we know we cannot make a profit so we continue to the 
    // next element in the list which will be the new buying price
    if (sell_price < buy_price) {
      change_buy_index = true; 
      continue;
    }
    
    // if the selling price is greater than the buying price
    // we check to see if these two indices give us a better 
    // profit then what we currently have
    else { 
      var temp_profit = sell_price - buy_price;
      if (temp_profit > max_profit) { max_profit = temp_profit; }
      change_buy_index = false;
    }
    
  }
  
  return max_profit;
         
}

StockPicker([44, 30, 24, 32, 35, 30, 40, 38, 15]);