_inaccurate's blog

How to get the position of an element in a set after lower_bound?
By _inaccurate, history, 8 months ago, In English

Hello codeforces! I'm practicing an exercise. My idea is to get the position of the selected element after a binary search on a set. Can you guys help me?..

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8 months ago, # |
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Ordered set

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    8 months ago, # ^ |
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    what's that. Can you explain more for me. Thank you btw:3

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      8 months ago, # ^ |
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      Just google it. It's a policy based data structure that allows you to do exactly that. Normal std::sets only store the relative location to their back and front elements, (loosely analagous to a linked list), so it's not possible to get the position.

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    8 months ago, # ^ |
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    Hello! Are you from Ukraine? I think I've seen the name Maksym Shvedchenko several times.

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8 months ago, # |
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#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>

using namespace __gnu_pbds;

void solve() {
    using kth_set = __gnu_pbds::tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update>;
    
    kth_set s;
    s.insert(3); // {3}
    s.insert(1); // {1, 3};
    auto it = s.lower_bound(0); // 1
    cout << s.order_of_key(*it) << endl; // find({1, 3}, 1) == 0
    auto it2 = s.lower_bound(2); // 3
    cout << s.order_of_key(*it2) << endl; // find({1, 3}, 3) == 1
}
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8 months ago, # |
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Souce 1 — Ordered set
Source 2 — Blog

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