【？】codeforces721E Road to Home（DP+单调队列）

转换关系搞清楚了，但是关于那个单调性来处理DP的地方，还是不明白，应该是单调队列优化DP这个点不会，补一下代码，先学一下单调队列优化DP，

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#include<iostream>
#include<cstring>
#include<algorithm>
using namespace std;
const int maxn = 100005;
int f[maxn], g[maxn];

int main()
{
	int l, n, p, t;
	scanf("%d%d%d%d", &l, &n, &p, &t);
	int head =1, tail = 1;
	f[head] = 0, g[head] = -t;
	int ans = 0;
	for (int i = 1; i <= n; i++)
	{
		int x, y,ans1,ans2;
		scanf("%d%d", &x, &y);
		ans1 = ans2 = 0;
		if (head>1) head--;
		while (head <= tail&&g[head] + t <= y)
		{
			int nx = max(x, g[head] + t);
			int ny = y;
			if (f[head] + (ny - nx) / p > ans1)
			{
				ans1 = f[head] + (ny - nx) / p;
				ans2 = nx + (ny - nx) / p*p;
			}
			else if (f[head] + (ny - nx) / p == ans1)
			{
				ans2 = min(ans2,nx + (ny - nx) / p*p);
			}
			++head;
		}
		if (ans1 > ans)
		{
			ans = ans1;
			tail++;
			f[tail] = ans1;
			g[tail] = ans2;
		}
	}
	printf("%d\n", ans);
	return 0;
}