数位统计

 

void  init()
{
    f[ 0 ][ 0 ] = 1 ;
     int  i,j;
     for (i = 1 ;i <= 31 ;i ++ )
    {
        f[i][ 0 ] = f[i - 1 ][ 0 ];
         for (j = 1 ;j <= i;j ++ )
            f[i][j] = f[i - 1 ][j] + f[i - 1 ][j - 1 ];
    }
}
int  calc( int  x, int  k) // 计算[0x]里含有k个1的数的个数
{
     int  i;
     int  tol = 0 ,ans = 0 ;
     for (i = 31 ;i >= 0 ;i -- )
    {
         if (x & ( 1 << i))
        {
            x ^= ( 1 << i);
            ans += f[i][k - tol];
            tol ++ ;
             if (tol > k)
                 break ;
        }
            
    }
     if (tol + x == k)
        ans ++ ;
     return  ans;
}