分治递归思想
不断分割归并
#include <iostream>
using namespace std;
typedef long long LL;
const int N = 1e5 + 10;
int n;
int q[N],tmp[N];

LL merge_sort(int q[],int l,int r){
    if(l >= r)  return 0;
    int mid = l + r >> 1;
    LL res = merge_sort(q,l,mid) + merge_sort(q,mid + 1,r);
    int k = 0,i = l,j = mid + 1;
    while(i <= mid && j <= r){
        if(q[i] <= q[j])    tmp[k++] = q[i++];
        else{
            res += mid - i + 1;
            tmp[k++] = q[j++];
        }
    }
    while(i <= mid) tmp[k++] = q[i++];
    while(j <= r)   tmp[k++] = q[j++];
    for(i = l,j = 0;i <= r;++i,++j) q[i] = tmp[j];
    return res;
}

int main(){
    cin >> n;
    for(int i = 0;i < n;++i){
        cin >> q[i];
    }
    cout << merge_sort(q,0,n - 1) << endl;
    return 0;
}