I've been stuck on this problem for a while (Lazy Cows USACO 2005 US Open Gold) . I was able to get the recurrence correct . But my naive approach () TLEs .

First I compressed the positions by sorting them based on their column and breaking ties using the row number

Then I pre-processed 3 arrays

*costH*1[*i*][*j*] stores minimum area covered using a single horizontal rectangle , from*i*^{th}smallest position to*j*^{th}. If it's not possible (cows are not entirely up or entirely down) then its*INF*.*costH*2[*i*][*j*] stores minimum area covered using 2 horizontal rectangles . If it can be covered with a single rectangle then its*INF*.*costV*[*i*][*j*] stores minimum area covered using a single vertical rectangle . To be more precise , its a rectangle with top left coordinate at (1 , coordinate[i]) and bottom right at (2 , coordinate[j])

Using these 3 arrays

*DP*[*i*][*j*] stores the minimum area covered by *i* rectangles for a prefix area till *j*^{th} column.

Base Case

*DP*[0][*j*] = *INF*

*DP*[1][*j*] = *min*(*costV*[0][*j*], *costH*1[0][*j*])

*DP*[*i*][0] = 0

My Submission : Commented Code

Can someone help me in optimising this recurrence relation.

### Update

I would like to thank send_nodes for helping me in solving this problem .

Auto comment: topic has been updated by bhishma (previous revision, new revision, compare).As

jincreases, what happens to the value forkwhich attains the minimum?I feel that they should increase , because

cost[k][j] ≤cost[k][j+ 1] for a particulark. AlsoDP[i- 1][k] +cost[k+ 1][j] ≤DP[i- 1][k] +cost[k+ 1][j+ 1] for 0 ≤k<j, so the minimum index increases asjincreases .Yeah, so you can use divide and conquer opt. to solve it.

Sorry I'm not aware of divide and conquer optimisation . Can you please explain it briefly .

So first, you should do an outer loop on i, and an inner loop on j to compute the DP (using your variable naming above). Naive way is O(N^2), but D&C can do it in O(N log N).

Idea is that if you want to compute dp[i][x] for all x in a range [l,r], and you know that the possible values for k (again using your naming above) lie in the range [a,b], you should compute dp[i][(l+r)/2] first and note the k that attained this minimum, let it be p. Then, you can compute dp[i][x] for all x in range [l,(l+r)/2 -1] now using the k-range [a,p] and dp[i][x] for all x in range [(l+r)/2 + 1, r] using k-range [p,b]. It works because of that monotonicity condition.

There is a problem . Our assumption that

kincreases asjis wrong .For this test case (TC 6)

test caseUsing the naive approach the

karray looks like this`[-1, -1, 0, 2, 2, 1, 1, 6, 6, 8, 9, 8, 11, 11, 11 ...`

fori= 4.What should I do now ?

Oops, you're right. Look at my comment below.

In the second base case shouldn't it be: dp[1][j]=min(costV[0][j],costH1[0][j]) instead of i?

Thanks for the correction . I have updated it.

Auto comment: topic has been updated by bhishma (previous revision, new revision, compare).Ok, I finally managed to get this problem accepted. I used a different dp state:

(index, number of rectangles left to place, is there a horizontal rectangle ending at (1,index), is there a horizontal rectangle ending at (2,index), is there a vertical rectangle ending at (1,index) and (2,index) ).

It has N*K*2*2*2 states. For transitions, to get to some state, you can extend the top rectangle, extend the bottom rectangle, extend top and bottom rectangle, extend a vertical rectangle, create a new top rectangle, create a new bottom rectangle, create a top and a bottom rectangle, create a vertical rectangle, discontinue a top rectangle, discontinue a bottom rectangle, disc. a top and bottom rectangle, disc. a vertical rectangle.

Time complexity was O(N*K*(2^3)^2) for my solution. Cool problem, thanks for sharing :D

What do you mean by discontinuing a rectangle .

at index

i- 1 we had some rectangle (top, bottom, top&bottom, or vertical), and at indexi, we don't put any rectangle in its place (we don't create a new rectangle nor do we continue it). I guess better word is "ending" a rectangle.Thanks a lot I was able to solve the problem.

Auto comment: topic has been updated by bhishma (previous revision, new revision, compare).