求逆序对的个数

#include<bits/stdc++.h>

using namespace std;

const int N=1e6+10;

int a[N];

int tmp[N];

typedef long long LL;

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(int i=l,j=0;i<=r;i++,j++) q[i]=tmp[j];

return res;

}

int main()

{

int n;

cin>>n;

for(int i=0;i<n;i++)

{

cin>>a[i];

}

cout<<merge_sort(a,0,n-1)<<endl;

return 0;

}