逆序对的数量（归并排序）

    while(i<=mid&&j<=r){
        if(q[i]<=q[j]) {res[k++]=q[i++];}
        else {
            ans+=mid-i+1;
            res[k++]=q[j++];
        }
    }

#include<iostream>
#include<algorithm>
using namespace std;
const int maxn=1e6+10;
int q[maxn];
int n;
int res[maxn];
long long merge_sort(int *q,int l,int r)
{
    if(l>=r) return 0;
    int mid=l+r>>1;
    long long ans=0;
    ans=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]) {res[k++]=q[i++];}
        else {
            ans+=mid-i+1;
            res[k++]=q[j++];
        }
    }
    while(i<=mid) res[k++]=q[i++];
    while(j<=r) res[k++]=q[j++];
    for(int i=l,j=0;i<=r;i++,j++) q[i]=res[j];
    return ans;
}
int main()
{
    scanf("%d",&n);
    for(int i=0;i<n;i++) scanf("%d",&q[i]);
    printf("%lld\n",merge_sort(q,0,n-1));
    return 0;
}