【树状数组（逆序数）】hdu 1394 Minimum Inversion Number

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;

const int NM=5005;
int a[NM],c[NM];

int lowbit(int x)
{
return x&(-x);
}

void add(int x)
{
while(x<NM)
{
c[x]++;
x+=lowbit(x);
}
}

int getsum(int x)
{
int ans=0;
while(x>0)
{
ans+=c[x];
x-=lowbit(x);
}
return ans;
}

int main()
{
int n,i,t,mmin,res;
while(scanf("%d",&n)!=EOF)
{
for(i=1;i<=n;i++){
scanf("%d",&t);
a[i]=++t;
}

memset(c,0,sizeof(c));
res=0;
for(i=1;i<=n;i++)
{
add(a[i]);
res+=i-getsum(a[i]);
}
mmin=res;
for(i=2;i<=n;i++)
{
res=res+n-2*a[i-1]+1;  //公式
if(res<mmin) mmin=res;
}
printf("%d\n",mmin);
}
return 0;
}