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countInversions.cpp
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countInversions.cpp
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#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int DivideConquer(int i, int j, std::vector<int> &arr) {
int res = 0;
if (j - i <= 0) return 0;
int mid = (i + j) / 2;
res += DivideConquer(i, mid, arr);
res += DivideConquer(mid + 1, j, arr);
for (int k = mid + 1; k <= j; k++) {
auto it = upper_bound(arr.begin() + i, arr.begin() + mid + 1, arr[k]);
if (it != arr.begin() + mid + 1) {
res += (arr.begin() + mid - it) + 1;
}
}
sort(arr.begin() + i, arr.begin() + j + 1, [] (int a, int b) { return a < b; });
return res;
}
int countInversions(std::vector<int> arr)
{
return DivideConquer(0, arr.size() - 1, arr);
}
int main() {
vector<int> arr = {4,6,2,9};
cout <<DivideConquer(0, arr.size() - 1, arr);
return 0;
}