[luoguP1868] 饥饿的奶牛（DP）

传送门

先把所有区间按照左端点排序

f[i]表示区间0~i的最优解

#include <cstdio>
#include <iostream>
#include <algorithm>
#define max(x, y) ((x) > (y) ? (x) : (y))

int n, v;
int f[3000001];

struct node
{
	int x, y;
}p[150001];

inline int read()
{
	int x = 0, f = 1;
	char ch = getchar();
	for(; !isdigit(ch); ch = getchar()) if(ch == '-') f = -1;
	for(; isdigit(ch); ch = getchar()) x = (x << 1) + (x << 3) + ch - '0';
	return x * f;
}

inline bool cmp(node a, node b)
{
	return a.x < b.x;
}

int main()
{
	int i, j = 1;
	n = read();
	for(i = 1; i <= n; i++) p[i].x = read(), p[i].y = read(), v = max(v, p[i].y);
	std::sort(p + 1, p + n + 1, cmp);
	for(; !p[j].x; j++) f[p[j].y] = p[j].y + 1;
	for(i = 1; i <= v; i++)
	{
		f[i] = max(f[i], f[i - 1]);
		for(; p[j].x == i; j++) f[p[j].y] = max(f[p[j].y], f[i - 1] + p[j].y - p[j].x + 1);
	}
	printf("%d\n", f[v]);
	return 0;
}