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;
}