hdu2838 Cow Sorting(树状数组)

树状数组求逆序对

#include<iostream>
#include<cstring>
#include<cstdio>
#include<cmath>
#include<algorithm>
#include<cstdlib>
#include<map>
#include<queue>
#include <deque>
#include <list>
#include <ctime>
#include <stack>
#include <vector>
#include<set>
#define Maxn 100005
#define MOD
typedef long long ll;
#define FOR(i,j,n) for(int i=j;i<=n;i++)
#define DFR(i,j,k) for(int i=j;i>=k;--i)
#define lowbit(a) a&-a
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
const int inf = 0x3f3f3f3f;
const double pi = acos(-1.0);
using namespace std;
ll c[Maxn],a[Maxn];
void add(int i,int val)
{ while(i<=Maxn)
{ c[i]+=val;
i+=lowbit(i);
}
}
int sum(int i)
{ int s=0;
while(i>0)
{ s+=c[i];
i-=lowbit(i);
}
return s;
}
int main()
{ int n;
while(~scanf("%d",&n))
{ ll ans=0;
memset(c,0,sizeof c);
FOR(i,1,n) scanf("%d",&a[i]);
FOR(i,1,n)
{
ans +=(i-1-sum(a[i]))*a[i];
add(a[i],1);
}
memset(c,0,sizeof c);
DFR(i,n,1)
{
ans += a[i]*sum(a[i]-1);
add(a[i],1);
}

printf("%I64d\n",ans);

}
return 0;
}