Inversions_PBDS

#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;

#define ordered_set tree<int, null_type,less<int>, rb_tree_tag,tree_order_statistics_node_update>

long long howManySwaps(vector<int> arr){
    int n = arr.size();
    long long ans = 0;
    ordered_set o_set;
    for(int i=0;i<n;i++){
        int x = o_set.order_of_key(arr[i]);
        ans = ans + (int)(o_set.size())-x;
        o_set.insert(arr[i]);
    }
    return ans;
}