莫队

```cpp
struct query
{
	int l,r,id,ans;
}q[maxn];

bool cmp1(const query &q1, const query &q2)
{
	if (belong[q1.l] != belong[q2.l]) return belong[q1.l] < belong[q2.l];
	else return q1.r < q2.r;
}

bool cmp2(const query &q1, const query &q2)
{
	return q1.id < q2.id;
}

void ins(int x)
{
	cnt[x] ++;
	if (cnt[x] % 2 == 0) ans++;
	else if (cnt[x] != 1) ans--;
}

void del(int x)
{
	cnt[x] --;
	if (cnt[x] != 0)
	{
		if (cnt[x] % 2 == 0) ans++;
		else ans--;
	}
}

int main()
{
	cin >> n >> m;
	for (int i=1;i<=n;i++)
		cin >> a[i];
	for (int i=1;i<=m;i++)
	{
		cin >> q[i].l >> q[i].r;
		q[i].id = i;
	}
	int s = sqrt(n);
	for (int i=1;i<=n;i++)
		belong[i] = i/s+1;
	sort(q+1,q+m+1,cmp1);	
		
	for (int i=q[1].l;i<=q[1].r;i++)
		ins(a[i]);
	q[1].ans = ans;
	for (int i=2;i<=m;i++)//O(Nsqrt(N))
	{
		int l1=q[i-1].l,r1=q[i-1].r;
		int l2=q[i].l,r2=q[i].r;
		
		if (l1 < l2)
			for (int i=l1;i<l2;i++)
				del(a[i]);
		else
			for (int i=l2;i<l1;i++)
				ins(a[i]);
		
		if (r1 < r2)
			for (int i=r1+1;i<=r2;i++)
				ins(a[i]);
		else
			for (int i=r2+1;i<=r1;i++)
				del(a[i]);

q[i].ans = ans;
	}
	sort(q+1,q+m+1,cmp2);
	for (int i=1;i<=m;i++)
		cout << q[i].ans << "\n";
		
	return 0;
}
```