洛谷3084 [USACO13OPEN]照片Photo

原题链接

神仙(DP)啊。。。
题解请移步隔壁大佬的博客(QAQ)

#include<cstdio>
using namespace std;
const int N = 2e5 + 10;
int L[N], R[N], q[N], f[N];
inline int re()
{
	int x = 0;
	char c = getchar();
	bool p = 0;
	for (; c < '0' || c > '9'; c = getchar())
		p |= c == '-';
	for (; c >= '0' && c <= '9'; c = getchar())
		x = x * 10 + c - '0';
	return p ? -x : x;
}
inline int minn(int x, int y)
{
	return x < y ? x : y;
}
inline int maxn(int x, int y)
{
	return x > y ? x : y;
}
int main()
{
	int i, j = 0, n, m, x, y, l = 1, r = 0;
	n = re();
	m = re();
	for (i = 2; i <= n + 1; i++)
		R[i] = i - 1;
	for (i = 1; i <= m; i++)
	{
		x = re();
		y = re();
		R[y] = minn(R[y], x - 1);
		L[y + 1] = maxn(L[y + 1], x);
	}
	for (i = n; i; i--)
		R[i] = minn(R[i], R[i + 1]);
	for (i = 2; i <= n + 1; i++)
		L[i] = maxn(L[i], L[i - 1]);
	for (i = 1; i <= n + 1; i++)
	{
		for (; j <= R[i]; j++)
			if (f[j] >= 0)
			{
				for (; l <= r && f[q[l]] < f[j]; r--);
				q[++r] = j;
			}
		for (; l <= r && q[l] < L[i]; l++);
		if (l <= r)
			f[i] = f[q[l]] + 1;
		else
			f[i] = -1;
	}
	printf("%d", f[n + 1] > 0 ? f[n + 1] - 1 : -1);
	return 0;
}