HDU 4911: Inversion

Posted 2020-12-03 zjnu

#include <bits/stdc++.h>
using namespace std;
const int N = 100005, INF = INT_MAX;
int n, k, A[N], L[N], R[N];
long long cnt;
void Merge(int p, int q, int r) {
  int n1 = q - p + 1, n2 = r - q;
  for (int i = 0; i < n1; i++)
   L[i] = A[i + p];
  for (int i = 0; i < n2; i++)
   R[i] = A[i + q + 1];
  L[n1] = INF;
  R[n2] = INF;
  for (int i = 0, j = 0, k = p; k <= r; k++)
   if (L[i] <= R[j])
     A[k] = L[i++];
   else {
     A[k] = R[j++];
     cnt += n1 - i;
   }
}
void MergeSort(int p, int r) {
  if (p < r) {
   int q = (p + r) / 2;
   MergeSort(p, q);
   MergeSort(q + 1, r);
   Merge(p, q, r);
  }
}
int main() {
  ios::sync_with_stdio(false);
  while (cin >> n >> k) {
   cnt = 0;
   for (int i = 0; i < n; i++)
     cin >> A[i];
   MergeSort(0, n - 1);
   cout << max(0LL, cnt - k) << endl;
  }
  return 0;
}