freda的越野跑c++

#include <iostream>
using namespace std;
int a[100001];
int ta[100001];
long long int ans = 0;
void merge(int left, int right, int middle) {
  int i, j, index1, index2;
  for (j = left; j <= right; ++j)
    ta[j] = a[j];
  index1 = left;
  index2 = middle + 1;
  i = left;
  while (index1 <= middle && index2 <= right) {
    if (ta[index1] >= ta[index2]) {
      a[i] = ta[index1];
      ++i;
      ++index1;
    } else {
      a[i] = ta[index2];
      ans += middle - index1 + 1; // 这个右子列元素超过了左子列中剩余的元素
      ++i;
      ++index2;
    }
  }
  while (index1 <= middle) {
    a[i] = ta[index1];
    ++i;
    ++index1;
  }
  while (index2 <= right) {
    a[i] = ta[index2];
    ++i;
    ++index2;
  }
}
void mergesort(int left, int right) {
  int middle;
  if (left < right) {
    middle = (left + right) / 2;
    mergesort(left, middle);
    mergesort(middle + 1, right);
    merge(left, right, middle);
  }
}
int main() {
  int n;
  cin >> n;
  for (int i = 0; i < n; ++i) {
    scanf("%d", &a[i]);
  }
  mergesort(0, n - 1);
  cout << ans;
  // system("pause");
  return 0;
}