Editorial for DMOPC '19 Contest 2 P3 - Selection

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Author: KevinWan

For subtask 1, a simple brute force algorithm will suffice. For each query, we can sort the subarray, take the cth best item, and then revert the array to its original state.

Time complexity: \mathcal{O}(MN \log N)

For subtask 2, we can build a Binary Indexed Tree (BIT) over our array and update it accordingly. For each query, we use our BIT to find the number of 1s in our subarray. If this is more than c, then the cth best item has goodness 1. Otherwise, it has goodness 0.

Time complexity: \mathcal{O}((M+N) \log N)

For subtask 3, we instead build a BIT for the number of elements for each possible g_i. Similar to subtask 2, we can use our BIT's to find for each possible g_i, the number of elements that have g_i as its goodness. The cth best item is thus the highest possible k such that the number of elements with a value greater than or equal to k is more than c, which can be checked with our BITs.

Time complexity: \mathcal{O}((M \max(g_i)+N) \log N)


  • -1
    Swarley  commented on Oct. 21, 2019, 11:46 a.m. edit 2

    A segment tree for number frequency would suffice as well. O(20mlog(n))