树状数组

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

int sum(int x)
{
    int ret = 0;
    while(x > 0)
    {
        ret += tmp[x]; x -= lowbit(x);
    }
    return ret; 
} 

void add(int x, int d)
{
     while(x <= ｎ）
    {
         tmp[x] += d; x += lowbit(x);
    }
}
void init()
{
   for(int i = 1; i <= n; ++i)
      add(i,a[i]);
}