This is a famous software engineering interview question from Google. I thought I’d provide my explanation that is borderline pedantic. Let’s solve this problem as naively as possible then again with nothing more than highschool maths.

The Problem

There are a few different versions of this question. Sometimes it is crystal balls, or eggs or something else.

There is a 100 story building. We have two magic glass orbs. The orbs will only break at or above floor X. We need to find what floor X is, in as few drops as possible. No matter how many times we drop an orb, it will only break at or above floor X. We are given two orbs. What is the fewest drops to find the floor no matter what the floor is? We are trying to minimize the worst case here.

The Worst Case Scenario

What make this problem unique is that we have limited failed tries, 2, and must optimize for the worst case-scenario more than the average runtime. I think that is an important concept to highlight.

Naive

Simplify the problem

If we only had one ball, then we’d just have to start at the bottom floor and work our way up. If we had infinite balls, we could just do a binary search and we’d only need log(n) balls at most (7 balls for 100 floor) But 2? HMM. So obviously, once the second orb breaks, we’ll be falling back to the floor-by-floor algorithm.

First thoughts

Most will first think about cutting the floors in half. So let’s do that. Give a building with N floor, n=100 here, we start at floor 50 (n/2), if the ball breaks, then we check from 1-49. If the ball doesn’t break, then we check 51-100. So we broke the building into 2 sections of size 50. The worst case is 50+2-1=51.

We might realize we could break the building up into differently sized sections.

If we break it into 4 sections of 25 then our worst case is 25+4-1 = 28 (27 if we use inference).

WE can see the pattern and make a formula. sections, S, and section size B. So tries, T make is

$$where \ S \ and \ B \ are \ factors \ of \ N$$ $$T=S+B-1$$

So we can manually find a good solution

2,50 = 51
4,25 = 28
5,20 = 24
10,10 = 19

After 10,10 it just gets worse again. You might even realize since S and B are the same, we could think of that as sqrt n. Which is a good answer. Not the ideal answer but a decent one.

Our generalized worst case would be to jump by

$$ \sqrt(n) $$

So for any building size N. We can solve it in

$$ 2 \sqrt(n) - 1$$

tries. For N=100, that is 19 (18 with inference).

If X = 99 then we try 10,20,30,40,50,60,70,80,90,100,91,92,93,94,95,96,97,98.

The Quadratics

The ideal solution comes from understanding that as we check the top of each section, we have already dropped for each previous section.

For example, if we jump by 10 each time then at floor 30 we have already jumped 2 times, 10,20. If the ball breaks we still have to check 10 places, 21-30 which make our total 12, not 10. Ideally that number would be consistent. We need to find an equalibrium. We would jump by 10 then by 9 because then we’d have a max of 10 jumps. We can try this but 10 doesn’t quite work. We end at 55. So we need a bigger number. 55 is the sum of 1 to 10. We could manually find that 14 + 13 + 12 + 11.. is 104 which is enough! But as computer scientists, we should look for the generalized solution.

So each jump must be one less than the previous. All jumps must add up to 100 (or more). We get the following formula that you’ll see in other blog posts.

$$n+n-1+n-2+n-3+n-4…=100$$

I’ve seen this formula on many blog posts that don’t explain it clearly for the beginners. This formula is really the same as

$$ \sum_{i=1}^n i $$

Which just means “Add all numbers from 1 to N”.

With a little thinking we realize that is is going to go down to 1. So really we are adding 1 to N and the sum being 100. We were just defining it in a backwards sense.

So we can figure out the sum off all numbers 1 to n with the following formula that is easy to figure out on your own.

Adding 1 to 10, we can make 10 pairs that add up to 10. There are always $ n \over 2 $ pairs. The middle number doesn’t pair up so we add that.

10 + 0 = 10 9 + 1 = 10 8 + 2 = 10 7 + 3 = 10 6 + 4 = 10

$$ n (n + 1) \over 5 $$

Which can be simplified to

$$ .5n^2 + .5n = floors $$

Or in our case of 100 floors

$$ .5n^2 + .5n = 100 $$

Then make it all equal 0 for balance

$$ .5n^2 + .5n - 100 = 0 $$

BOOM! we have a quadratic formula we can solve.

Remember your quadratics? Yeah! That’s just a quadratic formula that we can solve!

$$(ax^2 + bx + c = 0)$$

Our formula is

$$a = .5$$ $$b = .5$$ $$c = -100$$

and the quadratic formula.

$$ x = {-b \pm \sqrt{b^2-4ac} \over 2a} $$

So solve this formula (just use an online solver)

$$x = {-.5 \pm \sqrt{.5^2-4(.5)(-100)} \over 2*.5} $$

$$x = {-.5 \pm \sqrt{(.25)-(-200)} \over 1} $$

$$x = {-.5 \pm \sqrt{200.25}} $$

So if we do that, we get 13.650971698085 for the positive solution. We can ignore the negative solution.

Now, we can only have a whole floor number so we round to the nearest number and that is 14.

I used demos to chart the formula out. You can see the chart here. Or enjoy the screenshot below.

Parabola touching 13.6 on the x axis

So n=14. Which means that we start at floor n, 14 then move n-1 floors to 27. We continue moving one less floor each time.

I.E if we try 14,27 then the ball breaks at 27, we have to try 15-26 which is 12 tries plus the two from before = 14! Each time we have one less try for the in-between floors.

14,27,39,50,60,69,77,84,90,95,99,100

The Code

We have solved the problem and did the math. Now it is time to translate this into code. I’m going to write this in javascript, the language I typically use for interviews.

We don’t need to solve the quadratic equation in code. Remember the reverse formula? n+n-1+n-2...? We can just start at the top and subtract 1, then 2, then 3 etc. Until we get to 0 or less.

function findFloor(floors) {
  let steps = [];
  let step = 1;

  while (floor > 0) {
    steps.unshift(floor);
    floor = floor - step;
    steps += 1;
  }
  return steps;
}

You’ll notice this gives us a different answer! We have a different sequence of floors.

[9, 22, 34, 45, 55, 64, 72, 79, 85, 90, 94, 97, 99, 100];

Still, the worst case is 14. This one is ever so slightly more efficient because if the ball breaks on floor 9, then we only have 9 tries, everything else is 13 or 14. (13 because if we try 97 then 99 breaks we can infer that 98 is the floor).

The Same Answer

That was just a fun detour. We can code it to get the same floors. We would have to solve the quadratic formula THEN start at that floor and jumping by one less each time.

function findFloor(n) {
  const a = 0.5;
  const b = 0.5;
  const c = -n;
  // solve quadratic
  let floor = Math.ceil(-b + Math.sqrt((b ^ 2) - 4 * a * c));
  // walk the floors
  let steps = [floor];
  let step = floor;
  while (floor <= n) {
    floor += --step;
    steps.push(floor >= n ? n : floor);
  }
  return steps;
}

Conclusion

I know that was a REALLY long explanation but I think it was much more thorough than any other blog post about it.