J - Wormholes Time Limit:3000MS     Memory Limit:0KB     64bit IO Format:%lld & %llu Submit Status Description In the year 2163, wormholes were discovered. A wormhole is a subspace tunnel through space and time connecting two star systems. Wormholes…

Wormholes Time Limit: 2000MS   Memory Limit: 65536K Total Submissions: 49962   Accepted: 18421 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…

题目链接:http://poj.org/problem?id=3259 Wormholes Time Limit: 2000MS   Memory Limit: 65536K Total Submissions: 55082   Accepted: 20543 Description While exploring his many farms, Farmer John has discovered a number of amazing wormholes. A wormhole is ver…

MARK 用spfa判断是否存在负环 判断是否存在负环的方法有很多, 其中用spfa判断的方法是:如果存在一个点入栈两次,那么就存在负环. 细节想想确实是这样,按理来说是不存在入栈两次的如果边权值为正的话 这个算法是O(N*M) 还有一种方法是直接用bellman-ford,虽说spfa也就是bellman-ford+FIFO队列 而且bellman-ford还可以计算负环的值 顺手附上代码好了: for(int i=0;i<n;i++) d[i]=INF;//初始化 d[0]=0; for(i…

Wormholes Time Limit: 2000MS   Memory Limit: 65536K Total Submissions: 36852   Accepted: 13502 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…

spfa     (Shortest Path Faster Algorithm) 是一种单源最短路径的算法,基于Bellman-Ford算法上由队列优化实现. 什么是Bellman_Ford,百度内食用QWQ 也就是说,Bellman_Ford是一种无脑,疯狂松弛的算法.其复杂度为O(nm),可想而知,对于上万的数据会炸的一塌糊涂... 相对而言,SPFA显得就没那么无脑了. 在Bellman_Ford算法上,我们找到了一种优化松弛的方法:对于其子边没有进行松弛的松弛操作,当前操作不可能得出正…

经过笔者的多次实践(失败),在此温馨提示:用SPFA判负环时一定要特别小心! 首先SPFA有BFS和DFS两种实现方式,两者的判负环方式也是不同的.       BFS是用一个num数组,num[x]表示从1到x的最短路径包含的边数,当执行松弛操作d[y]=d[x]+w时,同样更新num[y]=num[x]+1,若此时发现num[y]>=n,则图中有负环(显然,n个点n条不重的边,必定又环).DFS则是换了一种思路:把d数组的初值置为0,这样就能保证走过的路径和一直为负,排除了大量无关路径.但是…

题目链接:poj3259 Wormholes 题意:虫洞问题,有n个点,m条边为双向,还有w个虫洞(虫洞为单向,并且通过时间为倒流,即为负数),问你从任意某点走,能否穿越到之前. 贴个SPFA代码: #include<cstdio> #include<cstring> #include<algorithm> #include<iostream> #include<string> #include<stack> #include<…

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: 68097   Accepted: 25374 题目链接:http://poj.org/problem?id=3259 Description: While exploring his many farms, Farmer John has discovered a number of amazing wormholes. A wormhole is ve…