void add(int x, ll k)
{
	ll p = x;
	for (; x <= n; x += lowbit(x)) 
	{
		c1[x] += k;
		c2[x] += k * p;
	}
}
ll query(int x)
{
	ll ans = 0, p = x;
	for (; x ; x -= lowbit(x)) 
		ans += (p + 1) * c1[x] - c2[x];
	return ans;
}

原文 link

为什么公式里是 $\sum\limits_{i=1}^{p}d[i]\times i$ 但代码里是 c2[x] += k * p; 而不是 c2[x] += k * x; ?