(Sorry for my bad english!!)
Problem (QBSEGPAR — Vietnam Olympiad of Informatics 2005): For an integer array a1, a2, ..., an and a integer k. Determine the smallest integer M to divide the array into k non-empty parts so that the sum of the elements in each parts does not exceed M.
- Constraints: k ≤ n ≤ 15000, |ai| ≤ 30000.
UPD: For example, we have n = 5, k = 3, and the array is 4, 3, 2, 1, 5. Then here is one way to divide the array into 3 parts: [4, 3], [2], [1, 5]. You can also represent one way to divide the array into k parts by an array x1, x2, ..., xk (1 ≤ x1 < x2 < ... < xk = n) which part ith (i > 1) is subarray [xi - 1 + 1..xi] (first part is subarray [1..x1]).
I have an 
solution, which we do binary seaching for M, and dp to check if this M could satisfy (let f(i, j) = true means you could divide array a[1..i] into j parts so that the sum of the elements in each of j parts does not exceed M. Otherwise, f(i, j) = false. Then we can divide array a[1..n] into k satisfied parts if f(n, k) = true). Of course this couldn't pass all tests.
Then I checked for the solution. Here is it, we also do binary searching for M, but for each M, we do a different dp:
- fi = minimum number of parts which has sum of elements does not exceed M that you could divide from array a[1..i].
- gi = maximum number of parts which has sum of elements does not exceed M that you could divide from array a[1..i].
Required complexity is o(nlogn2). We can calculate f and g by o(nlogn) dp, which need segment tree or Fenwick tree (i haven't done it), or simple o(n2) dp. Okay, now here is the checking part, which is unclear to me: The solution said that array a[1..n] could divide into k satisfied parts if fn ≤ k ≤ gn. I think this might not be right. In my opinion, we can divide array a[1..n] into minimum fn satisfied parts, maximum gn satisfied parts, but it doesn't mean we can divide the array into k satisfied parts which fn ≤ k ≤ gn.
Could you tell me how to prove this solution right or wrong? Thanks for your help!
