Wednesday, June 25, 2014


ACM ICPC 2014 is over. The contest was incredibly tough: solving less than half the problems was enough to get a medal, and 20 teams didn't solve any of the problems (unfortunately including the UCT team, who got bogged down in problem K).

I managed to solve 5 problems during the contest (in 4:30, since I didn't wake up early enough to be in at the start), and I've solved one more since then. Here are my solutions, in approximately easy-to-hard order.

EDIT: note that I've made follow-up blog posts with more analysis.

D: game strategy

This was definitely the easiest one, and this is reflected in the scores. We can use DP to determine the set of states from which Alice can force a win within i moves. Let's call this w[i]. Of course, w[0] = {target}. To compute w[i+1], consider each state s that is not already a winning state. If one of Alice's options is the set S and \(S \subseteq w[i]\), then Alice can win from this state in i+1 moves.

We can repeat this N times (since no optimal winning sequence can have more than N steps), or just until we don't find any new winning states. We don't actually need to maintain all the values of w, just the latest one.

K: surveillance

Let's first construct a slower polynomial-time algorithm. Firstly, for each i, we can compute the longest contiguous sequence of walls we can cover starting at i. This can be done in a single pass. We sort all the possible cameras by their starting wall, and as we sweep through the walls we keep track of the largest-numbered ending wall amongst cameras whose starting wall we have passed. The camera with \(a > b\) are a bit tricky: we can handle them by treating them as two cameras \([a - N, b]\) and \([a, b + N]\).

Let's suppose that we know we will use a camera starting at wall i. Then we can use this jump table to determine the minimum number of cameras: cover as much wall starting from i as possible with one camera, then again starting from the next empty slot, and so on until we've wrapped all the way around and covered at least N walls. We don't know the initial i, but we can try all values of i. Unfortunately, this requires \(O(N^2)\) time.

To speed things up, we can compute accelerated versions of the jump table. If \(J_c[i]\) is the number of walls we can cover starting from i by using c cameras, then we already have \(J_1\), and we can easily calculate \(J_{a+b}\) given \(J_a\) and \(J_b\). In particular, we can compute \(J_2, J_4, J_8\) and so on, and them combine these powers of two to make any other number. This can then be used in a binary search to find the smallest c such that \(J_c\) contains a value that is at least N. The whole algorithm thus requires \(O(N\log N)\) time.

One catch that broke my initial submission is that if the jump table entries are computed naively, they can become much bigger than N. This isn't a problem, until they overflow and become negative. Clamping them to N fixed the problem.

C: crane balancing

This is a reasonably straightforward geometry problem, requiring some basic mechanics. Let the mass of the crane be \(q\), let the integral of the x position over the crane be \(p\), let the base be the range \([x_0, x_1]\), and let the x position of the weight be \(x\). The centre of mass of the crane is \(\frac p q\), but with a weight of mass \(m\), it will be \(\frac{p+mx}{q+m}\). The crane is stable provided that this value lies in the interval \([x_0, x_1]\). This turns into two linear inequalities in \(m\). Some care is then needed to deal with the different cases of the coefficient being positive, negative or zero.

Computing the area and the integral of the x value is also reasonably straightforward: triangulate the crane by considering triangles with vertices at \((0, i, i+1)\) and computing the area (from the cross product) and centre of mass (average of the vertices) and then multiply the area by the x component of the centre of mass to get the integral.

I prefer to avoid floating point issues whenever possible, so I multiplied all the X coordinates by 6 up front and worked entirely in integers. This does also cause the masses to be 6 times larger, so they have to be adjusted to compensate.

E: maze reduction

This is effectively the same problem I set in Topcoder SRM 378 (thanks to ffao on Topcoder for reminding me where I'd seen this). If you have an account on Topcoder you can read about an alternative solution in the match analysis, in which it is easier to compute worst-case bounds.

The approach I used in this contest is similar in concept to D, in that you incrementally update a hypothesis about which things are distinguishable. We will assign a label to every door of each chamber. Two doors are given the same label if we do not (yet) know of a way to distinguish them. Initially, we just label each door with the degree of the room it is in, since there is nothing else we can tell by taking zero steps.
Now we can add one inference step. Going clockwise from a given door, we can explore all the corridors leading from the current chamber and note down the labels on the matching doors at the opposite end of each corridor. This forms a signature for the door. Having done this for each door, we can now assign new labels, where each unique signature is assigned a corresponding label. We can repeat this process until we achieve stability, i.e., the number of labels does not change. We probably ought to use each door's original label in the signature too, but my solution passed without requiring this.
Finally, two rooms are effectively identical if their sequence of door labels is the same up to rotation. I picked the lexicographically minimum rotation for each room and used this as a signature for the room.
I'm not sure what the theoretical work-case performance is, but I suspect it would be quite large. For a start, by algorithm requires \(O(NE\log N)\) time per iteration, which I suspect could be improved by using a hash table with some kind of rolling hash to rotation in O(1) time. I was surprised when my first submission ran in time.

B: buffed buffet

This one was deceptively sneaky, but the principles are reasonably simple. It's not too hard to guess that one will solve the continuous and discrete problems separately, and then consider all partitions between the two.
Let's start with the continuous problem, since it is a little easier. I don't have a formal proof, but it shouldn't be too hard to convince yourself that a greedy strategy works. We start by eating the tastiest food. Once it degrades to being as tasty as the second-tastiest food, we then each both, in the proportion that makes them decay at the same rate. In fact, at this point we can treat them as a combined food with a combined (slower) decay rate. We continue eating this mix until tastiness decays to match the third-tastiest, and so on. There are some corner cases that need to be handled if there are foods that don't decay.
Now what about the discrete case? Because the items have different weights, a greedy strategy won't work here, as with any knapsack problem. There is a reasonably straightforward \(O(DW^2)\) dynamic programming, where you consider each possible number of serving of each discrete food type, but this will be too slow. But there is some structure in the problem, so let's try to exploit it. Let's say that \(f(i)\) is the maximum tastiness for a weight of \(i\) using only the foods we've already considered, and we're now computing an updated \(f'\) by adding a new discrete food with weight \(w\), initial tastiness \(t\) and decay rate \(d\). For a given i, \(f'(i)\) clearly only depends on \(f(j)\) where \(i - j\) is a multiple of \(w\), so let's split things up by the remainder modulo \(i\). Fix a remainder \(r\) and let \(g(i) = f(iw + r)\) and \(g'(i) = f'(iw + r)\). Now we can say that
g'(i) &= \max_{0 \le j \le i}\big\{ g(j) + \sum_{n=1}^{i-j}(t - (n-1)d\big\}\\
&= \max_{0 \le j \le i}\big\{ g(j) + (i-j)t - \frac{(i-j-1)(i-j)}{2}\cdot d\big\}\\
&= \max_{0 \le j \le i}\big\{ g(j) + (i-j)t - \frac{i(i-1)+j(j+1)-2ij}{2}\cdot d\big\}\\
&= it - \frac{i(i-1)d}{2} + \max_{0 \le j \le i}\big\{ g(j)-\frac{j(j+1)d}{2} - jt + ijd\big\}\\
&= it - \frac{i(i-1)d}{2} + \max_{0 \le j \le i}\big\{ h(j) + ijd \big\}
Here we have defined \(h(j) = g(j)-\frac{j(j+1)d}{2} - jt\), which can be computed in constant time per \(j\).

The key observation is that we have turned the expression inside the maximum into a linear function of \(j\). Thus, if we plot \(h\) on a graph, only the upper convex hull is worth considering. We can maintain this upper hull as we increase \(i\) (remember, \(j \le i\)). The second observation is that the optimal choice of \(j\) will increase as \(i\) increases, because increasing \(i\) will increase the \(ijd\) more for larger values of \(j\). It follows that we can incrementally find the optimal \(j\) for each \(i\) by increasing the previous \(j\) along the upper hull until increasing it further would start decreasing the objective function. There are a few corner cases: the upper hull might not have any valid entries (because there might be no way to make up a weight of exactly \(jw+r\)), and we might have popped off the previous \(j\) as part of updating the hull. These just need careful coding. Another snag to watch out for is that if all the foods decay very fast, the optimal result may in fact overload a 32-bit integer in the negative direction.

The discrete part of the algorithm now requires O(DW), and the continuous part requires O(D\log D + W).

F: messenger (solved after contest)

This is a nasty problem mostly because of numerical stability problems. The problem itself is numerically unstable: a rounding error in measuring the total lengths of the paths is sufficient to change the answer between possible and impossible. It requires the black art of choosing good epsilons. I'll only describe the ideal case in which none of these nasty rounding problems occur.

Suppose we pick a point on both Misha and Nadia's path. In general this won't work, because the courier will arrive too late or too early at this point to intercept Nadia. Let L(A, B) be the number of time units that the courier is late when travelling from A to B. L can be late if the courier is early. Valid solutions are those where L = 0.

First, let's prove that L is an increasing function of A (where "increasing" means that A moves further along Misha's path). Suppose that the courier decided to, instead of moving straight to B, instead kept pace with Misha for a while, then went to B. Obviously this would mean arriving later (or at the same time). But this is equivalent to the arrival time if A increases. A similar argument shows that L is a decreasing function of B. In both cases, this is not strict.

This means that there can be at most \(O(N)\) pairs of segments for which L=0, rather than \(O(N^2)\), because we cannot move forward on one path and backwards on another. We can iterate over the candidate pairs by successively advancing either one segment or the other, by measuring L at pairs of segment endpoints.

Now we have reduced the problem to a single segment from each path. Given a point on one path, we can find the corresponding point on the other by solving what looks like a quadratic but which decays into a linear equation. Again, there are corner cases where the linear coefficient is also zero, in which case we have to break ties so as to minimise the travel time. Using this mapping function, we can identify the range of positions on Misha's path that correspond to valid positions on Nadia's path. I then used ternary search to find the optimal position along this segment. I didn't actually prove that the function is convex, but it seemed to work.

The solution I implemented that passed is actually rather messier than what I've described, and ran close to the time limit. I tried implementing it as described above but it hits assertions in my code due to numeric instabilities, but I think it should still be possible to fix it.


John Strawford said...

Great analysis!
I was wondering how you end up with O(N log N) for problem K.
If I understand well your approach, given a value c, find if Jc contains a value that is at least N costs O(N).
I know how to do it in O(N log N), which leads to an O(N log^2 N) time, but I can't get the idea for O(N).
Can you elaborate this, or point me some reference where I can read how to achieve it?


John Strawford said...
This comment has been removed by the author.
Zeta said...

Thank you, you're the first one posting an analysis for the problems of the world finals, I was a contestant and I didn't even understand what was the question in problem C (Crane Balancing), and now after seeing your tutorial I implemented it and it gives 0 .. 250 instead of 0 .. 1017 for the first sample test case? is there anything else to be done in that problem? thank you

Bruce Merry said...

@John: me saying "binary search" is slightly misleading. Instead, I work out the bits of c-1, from most significant to least significant. For each bit, I try adding a power of 2 (precomputed) to the candidate number I have, and if the resulting value still doesn't have a jump length of N then I keep the change, otherwise I revert it. It ends up testing exactly the values that a binary search does starting with endpoints at 0 and a large enough power of 2, but each midpoint is computed by adding a power of 2 to the lower bound.

@Zeta: I'm not sure. It's most likely just an implementation error: I had to go through a few iterations of bugfixing before I solved the sample.

John Strawford said...

Got it!
Thank Bruce! That was a nice trick

Unknown said...
This comment has been removed by the author.
Unknown said...
This comment has been removed by the author.
Maria Keet said...

Hi Bruce,

thanks for the analysis (though I probably will try first before reading the solutions in detail). I did (and solved) problem A on te plane, described here:


Zeta said...

@Bruce Merry thank you, but it was my centroid formula that was totally wrong, I got the right one from here
and got accepted...

stefano tommasini said...

Great analysis!
Can you post your code for B continues part?
Thank you very much

James Knot said...

I am amazed at your level of understanding of algortihms. I am trying to prepare for the next years ACM ICPC and was pracitising a few problems. I came across the shortest flight path problem from 2012. I havent been able to wrap my head around it. Would you mind giving me headstart on this?

Yahia Assalimy said...

Hi, anyone knows about problem I (sensor network)? I tried hard to solve but unfortunately I couldn't. Any help please.