双端队列（deque）

abc 247 - D - Cylinder

题目描述：

给定Q次操作，每次操作有如下两种选择

• 1 x c: Insert $c$ balls, with a number $x$ written on each of them, to the right end of the cylinder.

• 2 c: Take out the $c$ leftmost balls contained in the cylinder and print the sum of the numbers written on the balls that have been taken out.

数据范围：

1 ≤ Q ≤ 2×10⁵

0 ≤ x ≤ 10⁹

0 ≤ c ≤ 10⁹

解题思路：

由于x, c的范围过大，使用一维队列时，样例二会re, 因此该题可以考虑使用二维，直接记录数字和个数

使用双端队列可以方便的对队头元素进行删除和插入

val记录小球上的数字， cnt记录该段同种数字的小球的个数

当 c < cnt 时，注意要及时更新当前队头小球的数量

key code:

void solve()
{
	LL n;
	cin >> n;
	deque<pair<LL,LL> >dq;
	while(n --)
	{
		LL m;
		cin >> m;
		
		if(m == 1)
		{
			LL a, b;
			cin >> a >> b;
			dq.push_back({a, b});
		}
		else 
		{
			LL c, sum = 0;
			cin >> c;
			while(c)
			{
				auto it = dq.front();
				dq.pop_front();
				
				LL val = it.first, cnt = it.second;
				
				if(c < cnt)
				{
					cnt -= c;
					sum += c * val;
					c = 0;
					dq.push_front({val, cnt});
				}
				else 
				{
					c -= cnt;
					sum += val * cnt;
				}
			}
			cout << sum << endl;
		}
	}
}