IOI '19 - Shoes

#include "shoes.h"
#include <algorithm>
#include <vector>

using namespace std;

// Fenwick tree
void update(vector<int> &fenwick, int index, int value) {
    while (index < (int)fenwick.size()) {
        fenwick[index] += value;
        index += index & -index;
    }
}
int getSum(const vector<int> &fenwick, int index) {
    int sum = 0;
    while (index) {
        sum += fenwick[index];
        index -= index & -index;
    }
    return sum;
}
// Reverse fenwick tree for suffix sums
void updateR(vector<int> &fenwick, int index, int value) {
    update(fenwick, (int)fenwick.size()-index, value);
}
int getSumR(const vector<int> &fenwick, int index) {
    return getSum(fenwick, (int)fenwick.size()-index);
}

long long count_swaps(vector<int> s) {
    int n = (int)s.size() / 2;
    vector<vector<int>> nxt[2];
    nxt[0] = nxt[1] = vector<vector<int>>(1+n);
    for (int i = 2*n-1; i >= 0; --i) {
        if (s[i] < 0) nxt[0][-s[i]].push_back(i);
        else nxt[1][s[i]].push_back(i);
    }

    vector<int> fenwickMoved(1+2*n);
    vector<bool> paired(2*n);

    long long swaps = 0;
    for (int i = 0; i < 2*n; ++i) {
        if (paired[i]) continue;
        int v = abs(s[i]), side = s[i] > 0;
        int j = nxt[1-side][v].back();

        swaps += (j + getSumR(fenwickMoved, j+1)) - (i + getSumR(fenwickMoved, i+1));
        if (!side) --swaps; // do not need to swap with each other
        updateR(fenwickMoved, j+1, +1);

        paired[i] = true, paired[j] = true;
        nxt[0][v].pop_back(), nxt[1][v].pop_back();
    }
    return swaps;
}