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Sample Programming Interview Question

Writing coding interview questions hasn't made me rich. Maybe trading Apple stocks will.

Suppose we could access yesterday's stock prices as an array, where:

  • The indices are the time in minutes past trade opening time, which was 9:30am local time.
  • The values are the price in dollars of Apple stock at that time.

For example, if the stock cost $500 at 10:30am, stock_prices_yesterday[60] = 500.

Write an efficient function that takes stock_prices_yesterday and returns the best profit I could have made from 1 purchase and 1 sale of 1 Apple stock yesterday.

No "shorting"—you must buy before you sell. You may not buy and sell in the same time step (at least 1 minute must pass).

It is not sufficient to simply take the difference between the highest price and the lowest price, because the highest price may come before the lowest price. You must buy before you sell.

What if the stock value goes down all day? In that case, the best profit will be negative.

You can do this in time and space!

To start, try writing an example value for stock_prices_yesterday and finding the maximum profit "by hand." What's your process for figuring out the maximum profit?

The brute force approach would be to try every pair of times (treating the earlier time as the buy time and the later time as the sell time) and see which one is higher.

def get_max_profit(stock_prices_yesterday): max_profit = 0 # go through every time for outer_time in xrange(len(stock_prices_yesterday)): # for every time, go through every OTHER time for inner_time in xrange(len(stock_prices_yesterday)): # for each pair, find the earlier and later times earlier_time = min(outer_time, inner_time) later_time = max(outer_time, inner_time) # and use those to find the earlier and later prices earlier_price = stock_prices_yesterday[earlier_time] later_price = stock_prices_yesterday[later_time] # see what our profit would be if we bought at the # earlier price and sold at the later price potential_profit = later_price - earlier_price # update max_profit if we can do better max_profit = max(max_profit, potential_profit) return max_profit

But that will take time, since we have two nested loops—for every time, we're going through every other time. Can we do better?

Well, we’re doing a lot of extra work. We’re looking at every pair twice. We know we have to buy before we sell, so in our inner for loop we could just look at every price after the price in our outer for loop.

That could look like this:

def get_max_profit(stock_prices_yesterday): max_profit = 0 # go through every price (with its index as the time) for earlier_time, earlier_price in enumerate(stock_prices_yesterday): # and go through all the LATER prices for later_price in stock_prices_yesterday[earlier_time:]: # see what our profit would be if we bought at the # earlier price and sold at the later price potential_profit = later_price - earlier_price # update max_profit if we can do better max_profit = max(max_profit, potential_profit) return max_profit

What’s our runtime now?

Well, our outer for loop goes through all the times and prices, but our inner for loop goes through one fewer price each time. So our total number of steps is the sum n + (n - 1) + (n - 2) ... + 2 + 1, which is still time.

We can do better!

If we're going to do better than , we're probably going to do it in either or . comes up in sorting and searching algorithms where we're recursively cutting the set in half. It's not obvious that we can save time by cutting the set in half here. Let's first see how well we can do by looping through the set only once.

Since we're trying to loop through the set once, let's use a greedy approach, where we keep a running max_profit until we reach the end. We'll start our max_profit at $0. As we're iterating, how do we know if we've found a new max_profit?

At each iteration, our max_profit is either:

  1. the same as the max_profit at the last time step, or
  2. the max profit we can get by selling at the current_price

How do we know when we have case (2)?

The max profit we can get by selling at the current_price is simply the difference between the current_price and the min_price from earlier in the day. If this difference is greater than the current max_profit, we have a new max_profit.

So for every price, we’ll need to:

  • keep track of the lowest price we’ve seen so far
  • see if we can get a better profit

Here’s one possible solution:

def get_max_profit(stock_prices_yesterday): min_price = stock_prices_yesterday[0] max_profit = 0 for current_price in stock_prices_yesterday: # ensure min_price is the lowest price we've seen so far min_price = min(min_price, current_price) # see what our profit would be if we bought at the # min price and sold at the current price potential_profit = current_price - min_price # update max_profit if we can do better max_profit = max(max_profit, potential_profit) return max_profit

We’re finding the max profit with one pass and constant space!

Are we done? Let’s think about some edge cases. What if the stock value stays the same? What if the stock value goes down all day?

If the stock price doesn't change, the max possible profit is 0. Our function will correctly return that. So we're good.

But if the value goes down all day, we’re in trouble. Our function would return 0, but there’s no way we could break even if the price always goes down.

How can we handle this?

Well, what are our options? Leaving our function as it is and just returning zero is not a reasonable option—we wouldn't know if our best profit was negative or actually zero, so we'd be losing information. Two reasonable options could be:

  1. return a negative profit. “What’s the least badly we could have done?”
  2. throw an error. “We should not have purchased stocks yesterday!”

In this case, it’s probably best to go with option (1). The advantages of returning a negative profit are:

  • We more accurately answer the challenge. If profit is "revenue minus expenses", we’re returning the best we could have done.
  • It's less opinionated. We'll leave decisions up to our function’s users. It would be easy to wrap our function in a helper function to decide if it’s worth making a purchase.
  • We allow ourselves to collect better data. It matters if we would have lost money, and it matters how much we would have lost. If we’re trying to get rich, we’ll probably care about those numbers.

How can we adjust our function to return a negative profit if we can only lose money? Initializing max_profit to 0 won’t work...

Well, we started our min_price at the first price, so let’s start our max_profit at the first profit we could get—if we buy at the first time and sell at the second time.

min_price = stock_prices_yesterday[0] max_profit = stock_prices_yesterday[1] - stock_prices_yesterday[0]

But we have the potential for an index out of bounds error here, if stock_prices_yesterday has fewer than 2 prices.

We do want to throw an error in that case, since profit requires buying and selling, which we can't do with less than 2 prices. So rather than throwing a confusing index out of bounds error, let's explicitly catch that case and throw a more helpful error message:

if len(stock_prices_yesterday) < 2: raise IndexError('Getting a profit requires at least 2 prices') min_price = stock_prices_yesterday[0] max_profit = stock_prices_yesterday[1] - stock_prices_yesterday[0] # etc...

Ok, does that work?

No! max_profit is still always 0! What’s happening?

If the price always goes down, min_price is always set to the current_price. So current_price - min_price comes out to 0, which of course will always be greater than a negative profit.

When we’re calculating the max_profit, we need to make sure we never have a case where we try both buying and selling stocks at the current_price.

To make sure we’re always buying at an earlier price, never the current_price, let’s switch the order around so we calculate max_profit before we update min_price.

We'll also need to pay special attention to time 0. Make sure we don't try to buy and sell at time 0!

We’ll greedily walk through the array to track the max profit and lowest price so far.

For every price, we check if:

  • we can get a better profit by buying at min_price and selling at the current_price
  • we have a new min_price

To start, we initialize:

  1. min_price as the first price of the day
  2. max_profit as the first profit we could get

We decided to return a negative profit if the price decreases all day and we can't make any money. We could have thrown an error instead, but returning the negative profit is cleaner, makes our function less opinionated, and ensures we don't lose information.

def get_max_profit(stock_prices_yesterday): # make sure we have at least 2 prices if len(stock_prices_yesterday) < 2: raise IndexError('Getting a profit requires at least 2 prices') # we'll greedily update min_price and max_profit, so we initialize # them to the first price and the first possible profit min_price = stock_prices_yesterday[0] max_profit = stock_prices_yesterday[1] - stock_prices_yesterday[0] for index, current_price in enumerate(stock_prices_yesterday): # skip the first (0th) time # we can't sell at the first time, since we must buy first, # and we can't buy and sell at the same time! # if we took this out, we'd try to buy /and/ sell at time 0. # this would give a profit of 0, which is a problem if our # max_profit is supposed to be /negative/--we'd return 0! if index == 0: continue # see what our profit would be if we bought at the # min price and sold at the current price potential_profit = current_price - min_price # update max_profit if we can do better max_profit = max(max_profit, potential_profit) # update min_price so it's always # the lowest price we've seen so far min_price = min(min_price, current_price) return max_profit

time and space. We only loop through the array once.

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