逆序对的数量
第一次用int爆了
#include<iostream>
using namespace std;
const int N = 1e6 + 5;
int tmp[N];
int a[N];
long long merge_sort(int arr[], int l, int r){
if(l >= r) return 0;
int mid = (l + r) >> 1;
long long t = 0;//total
t += merge_sort(arr, l, mid);
t += merge_sort(arr, mid + 1, r);
long long ret = 0;
int k = 0, i = l, j = mid + 1;
while(i <= mid && j <= r)
if(arr[i] <= arr[j])
tmp[k++] = arr[i++];
else{
tmp[k++] = arr[j++];
ret += (mid - i + 1);
}
while(i <= mid) tmp[k++] = arr[i++];
while(j <= r) tmp[k++] = arr[j++];
for(int m = l, n = 0; m <= r; m++, n++) arr[m] = tmp[n];
t += ret;
return t;
}
int main(){
int n;
scanf("%d", &n);
for(int i = 0; i < n; i++)
scanf("%d", &a[i]);
long long k = merge_sort(a, 0, n - 1);
cout << k << endl;
return 0;
}