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| #include <vector>
using namespace std;
struct Seg {
int mi, ma, count;
Seg *l, *r;
Seg(vector<int> A, int left, int right) : count(0), l(nullptr), r(nullptr) {
mi = A[left], ma = A[right];
if (left == right) return;
int m = (left + right) / 2;
l = new Seg(A, left, m);
r = new Seg(A, m + 1, right);
}
~Seg() {
if(l) delete l;
if(r) delete r;
}
int query(int s, int e) {
if (mi > e or ma < s) return 0;
if (s <= mi and ma <= e) return count;
return (l ? l->query(s, e) : 0) + (r ? r->query(s, e) : 0);
}
void update(int n, int v) {
if (mi <= n and n <= ma) {
count++;
if (l) l->update(n, v);
if (r) r->update(n, v);
}
}
};
int countInversions(vector<int> array) {
if(array.empty()) return 0;
int res = 0, n = array.size();
vector<int> sorted = array;
sort(begin(sorted), end(sorted));
sorted.erase(unique(sorted.begin(), sorted.end()), sorted.end());
Seg *seg = new Seg(sorted, 0, sorted.size() - 1);
for (int i = 0; i < n; i++) {
int s = array[i] + 1, e = sorted.back();
if (s <= e)
res += seg->query(s, e);
seg->update(array[i], 1);
}
delete seg;
return res;
}
|
