题目链接:http://poj.org/problem?id=3259 题目大意是给你n个点,m条双向边,w条负权单向边.问你是否有负环(虫洞). 这个就是spfa判负环的模版题,中间的cnt数组就是记录这个点松弛进队的次数,次数超过点的个数的话,就说明存在负环使其不断松弛. #include <iostream> #include <cstdio> #include <cstring> #include <queue> using namespace st…

Description While exploring his many farms, Farmer John has discovered a number of amazing wormholes. A wormhole is very peculiar because it is a one-way path that delivers you to its destination at a time that is BEFORE you entered the wormhole! Eac…

Wormholes Time Limit: 2000MS Memory Limit: 65536K Total Submissions: 46123 Accepted: 17033 Description While exploring his many farms, Farmer John has discovered a number of amazing wormholes. A wormhole is very peculiar because it is a one-way path…

题意 : 给出 N 个点,以及 M 条双向路,每一条路的权值代表你在这条路上到达终点需要那么时间,接下来给出 W 个虫洞,虫洞给出的形式为 A B C 代表能将你从 A 送到 B 点,并且回到 C 个时间点之前,也就是时光倒流了 C 个时间并且此时你在 B 点,现在问你是否能够在图上的这些点中走,使得在某一个点刚好碰到之前的自己 分析 : 冷静分析一下,只要是有负权回路的某一条边属于虫洞的,那么肯定是能在这个环上一直绕,直到遇到之前的自己,如果将虫洞看作一条负权边的话,那么问题就变成了只要存在负…

Wormholes Time Limit: 2000MS   Memory Limit: 65536K Total Submissions: 36425   Accepted: 13320 Description While exploring his many farms, Farmer John has discovered a number of amazing wormholes. A wormhole is very peculiar because it is a one-way p…

http://poj.org/problem?id=3621 求一个环的{点权和}除以{边权和},使得那个环在所有环中{点权和}除以{边权和}最大. 0/1整数划分问题 令在一个环里,点权为v[i],对应的边权为e[i],  即要求:∑(i=1,n)v[i]/∑(i=1,n)e[i]最大的环(n为环的点数),  设题目答案为ans,  即对于所有的环都有 ∑(i=1,n)(v[i])/∑(i=1,n)(e[i])<=ans  变形得ans* ∑(i=1,n)(e[i])>=∑(i=1,n)(v…

题意:有向图判负环. 解题关键:spfa算法+hash判负圈. spfa判断负环:若一个点入队次数大于节点数,则存在负环.  两点间如果有最短路,那么每个结点最多经过一次,这条路不超过$n-1$条边.” 如果一个结点经过了两次,那么我们走了一个圈.如果这个圈的权为正,显然不划算:如果是负圈,那么最短路不存在:如果是零圈,去掉不影响最优值. 也就是说,每个点最多入队$n-1$次,可以想象一下,左边$n-1$个节点全部指向右边一个节点,遍历的顺序恰好与边权顺序相反. 负圈是指圈上的总和小于0 实际只…

原题链接:http://poj.org/problem?id=3259 题意 有个很厉害的农民,它可以穿越虫洞去他的农场,当然他也可以通过道路,虫洞都是单向的,道路都是双向的,道路会花时间,虫洞会倒退时间,问你这个农民可不可以回到起点,并且在他出发的时间之前. 题解 就虫洞用负边链接,然后判断是否有负环即可. 代码 #include<iostream> #include<cstring> #include<vector> #include<string> #…

POJ3259 :Wormholes 时间限制:2000MS 内存限制:65536KByte 64位IO格式:%I64d & %I64u 描述 While exploring his many farms, Farmer John has discovered a number of amazing wormholes. A wormhole is very peculiar because it is a one-way path that delivers you to its destin…

Wormholes Time Limit: 2000MS Memory Limit: 65536K Total Submissions: 42366 Accepted: 15560 传送门 Description While exploring his many farms, Farmer John has discovered a number of amazing wormholes. A wormhole is very peculiar because it is a one-way p…