原题链接 显然一个强连通分量里所有草场都可以走到,所以先用\(tarjan\)找强连通并缩点. 对于缩点后的\(DAG\),先复制一张新图出来,然后对于原图中的每条边的终点向新图中该边对应的那条边的起点连一条边,表示逆向走一次,且之后不会再逆向了. 最后在该图上跑\(SPFA\)求单源最长路即可. #include<cstdio> using namespace std; const int N = 1e5 + 10; struct eg { int x, y; }; eg a[N]; int…

P3119 [USACO15JAN]草鉴定Grass Cownoisseur 题目描述 约翰有\(n\)块草场,编号1到\(n\),这些草场由若干条单行道相连.奶牛贝西是美味牧草的鉴赏家,她想到达尽可能多的草场去品尝牧草. 贝西总是从1号草场出发,最后回到1号草场.她想经过尽可能多的草场,贝西在通一个草场只吃一次草,所以一个草场可以经过多次.因为草场是单行道连接,这给贝西的品鉴工作带来了很大的不便,贝西想偷偷逆向行走一次,但最多只能有一次逆行.问,贝西最多能吃到多少个草场的牧草. 输入输出格式…

P3119 [USACO15JAN]草鉴定Grass Cownoisseur 题目描述 In an effort to better manage the grazing patterns of his cows, Farmer John has installed one-way cow paths all over his farm. The farm consists of N fields, conveniently numbered 1..N, with each one-way co…

P3119 [USACO15JAN]草鉴定Grass Cownoisseur 题目描述 In an effort to better manage the grazing patterns of his cows, Farmer John has installed one-way cow paths all over his farm. The farm consists of N fields, conveniently numbered 1..N, with each one-way co…

http://www.lydsy.com/JudgeOnline/problem.php?id=3887|| https://www.luogu.org/problem/show?pid=3119 Description In an effort to better manage the grazing patterns of his cows, Farmer John has installed one-way cow paths all over his farm. The farm con…

屠龙宝刀点击就送 Tarjan缩点+拓扑排序 以后缩点后建图看n范围用vector ,或者直接用map+vector 结构体里数据要清空 代码: #include <cstring> #include <cctype> #include <cstdio> #include <vector> #include <queue> #define N 100005 using namespace std; vector<int>G[N]; i…

思路很乱,写个博客理一理. 缩点 + dp. 首先发现把一个环上的边反向是意义不大的,这样子不但不好算,而且相当于浪费了一次反向的机会.反正一个强连通分量里的点绕一遍都可以走到,所以我们缩点之后把一个强连通分量放在一起处理. 设$st$表示缩点之后$1$所在的点,设$f_{x}$表示从$st$走到$x$的最长链,$g_{x}$表示从$x$走到$st$的最长链,因为把一个$DAG$上的边反向一下并不会走重复的点,那么我们最后枚举一下边$(x, y)$,把它反向,这样子$f_{x} + g_{y}…

[USACO15JAN]草鉴定Grass Cownoisseur 题目描述 In an effort to better manage the grazing patterns of his cows, Farmer John has installed one-way cow paths all over his farm. The farm consists of N fields, conveniently numbered 1..N, with each one-way cow path…

草鉴定Grass Cownoisseur 题目链接 约翰有n块草场,编号1到n,这些草场由若干条单行道相连.奶牛贝西是美味牧草的鉴赏家,她想到达尽可能多的草场去品尝牧草. 贝西总是从1号草场出发,最后回到1号草场.她想经过尽可能多的草场,贝西在通一个草场只吃一次草,所以一个草场可以经过多次.因为草场是单行道连接,这给贝西的品鉴工作带来了很大的不便,贝西想偷偷逆向行走一次,但最多只能有一次逆行.问,贝西最多能吃到多少个草场的牧草. 如果没有逆行操作和回到1的限制,我们很容易想到一种方法: Tarj…

题目描述 In an effort to better manage the grazing patterns of his cows, Farmer John has installed one-way cow paths all over his farm. The farm consists of N fields, conveniently numbered 1..N, with each one-way cow path connecting a pair of fields. For…