HDU 3743 Frosh Week

HDU 3743 Frosh Week

树状数组求逆序对的简单应用。
以下是我的代码：

#include < iostream >
#include < algorithm >
#include < cstdio >
#include < cstring >
#define  lowbit(x) ((x)&(-(x)))
using   namespace  std;
const   int  kMaxn( 1000007 );

struct  Type
{
     int  value,num,weight;
};

bool  cmp1( const  Type  & a, const  Type  & b)
{
     return  (a.value < b.value);
}

bool  cmp2( const  Type  & a, const  Type  & b)
{
     return  (a.num < b.num);
}

int  n,bit[kMaxn];
Type r[kMaxn];
long   long  ans;

void  Add( int  pos, int  delta)
{
     for ( int  i = pos;i <= n;i += lowbit(i))
        bit[i] += delta;
}

int  Sum( int  pos)
{
     int  re( 0 );
     for ( int  i = pos;i > 0 ;i -= lowbit(i))
        re += bit[i];
     return  re;
}

int  main()
{
     // freopen("data.in","r",stdin);

     while (scanf( " %d " , & n) == 1 )
    {
         for ( int  i = 1 ;i <= n;i ++ )
        {
            r[i].num = i;
            scanf( " %d " , & r[i].value);
        }
        sort(r + 1 ,r + n + 1 ,cmp1);
         for ( int  i = 1 ;i <= n;i ++ )
            r[i].weight = i;
        sort(r + 1 ,r + n + 1 ,cmp2);

        ans = 0ll;
        memset(bit, 0 , sizeof (bit));
         for ( int  i = 1 ;i <= n;i ++ )
        {
            ans += i - Sum(r[i].weight - 1 ) - 1 ;
            Add(r[i].weight, 1 );
        }

        cout << ans << endl;
    }

     return   0 ;
}