题目链接:http://poj.org/problem?id=2135 今天学习最小费用流.模板手敲了一遍. 产生了一个新的问题:对于一条无向边,这样修改了正向边容量后,反向边不用管吗? 后来想了想,得出了个结论.路径所选的边只会包括正反中的一条. #include <iostream> #include <cstdio> #include <cstring> #include <vector> #include <queue> #include…

题目大意: 给定一个n个点m条边的无向图 求从点1去点n再从点n回点1的不重叠(同一条边不能走两次)的最短路 挑战P239 求去和回的两条最短路很难保证不重叠 直接当做是由1去n的两条不重叠的最短路 这样就变成了由1去n流量为2的最小费用流 #include <bits/stdc++.h> #define INF 0x3f3f3f3f using namespace std; ; int n,m; struct EDGE { int v,w,c,r; }; vector <EDGE>…

题目大意: 给你一个n个农场,有m条道路,起点是1号农场,终点是n号农场,现在要求从1走到n,再从n走到1,要求不走重复路径,求最短路径长度. 算法讨论: 最小费用最大流.我们可以这样建模:既然要求不能走重复路,就相当于每条边的容量是1,我们只可以单向流过容量为1的流量.但是要注意,对于每一条边来说, 它可能是去路的边,也可能是回路的边,所以这个图是个无向图.在加边的时候,两个方向的边都要加.所以要加两组的边,流量为1像正常一样加边就可以了. 然后我们考虑,求这个“环”就是相当于求从1..N的两…

POJ 2135 Farm Tour (网络流,最小费用最大流) Description When FJ's friends visit him on the farm, he likes to show them around. His farm comprises N (1 <= N <= 1000) fields numbered 1..N, the first of which contains his house and the Nth of which contains the b…

题目:id=2135" target="_blank">poj 2135 Farm Tour 题意:给出一个无向图,问从 1 点到 n 点然后又回到一点总共的最短路. 分析:这个题目不读细致的话可能会当做最短路来做,最短路求出来的不一定是最优的,他是两条分别最短,但不一定是和最短. 我们能够用费用流来非常轻易的解决,建边容量为1,费用为边权.然后源点s连 1 .费用0 .容量 2 ,n点连接汇点,容量2,费用0,,就能够了. 注意这个题目是无向图,所以要建双向边. AC…

[题意]给出一张无向图,从1开始到n,求两条没有公共边的最短路,使得路程总和最小 每条边的权值设为费用,最大流量设为1,然后就是从源点到汇点流量为2的最小费用流. 因为是规定了流量,新建一个源点和一个汇点,源点到结点1连一条最大流量为2,费用为0的边,结点N到汇点连一条最大流量为2,费用为0的边,这样就规定好流量了. #include<cstdio> #include<cstring> #include<cmath> #include<iostream> #…

When FJ's friends visit him on the farm, he likes to show them around. His farm comprises N (1 <= N <= 1000) fields numbered 1..N, the first of which contains his house and the Nth of which contains the big barn. A total M (1 <= M <= 10000) pa…

两条路不能有重边,既每条边的容量是1.求流量为2的最小费用即可. //#pragma comment(linker, "/STACK:1024000000,1024000000") #include<cstdio> #include<cstring> #include<cstdlib> #include<algorithm> #include<iostream> #include<sstream> #include…

题目链接:http://poj.org/problem?id=2135 Farm Tour Time Limit: 1000MS   Memory Limit: 65536K Total Submissions: 14821   Accepted: 5657 Description When FJ's friends visit him on the farm, he likes to show them around. His farm comprises N (1 <= N <= 1000…

Evacuation Plan Time Limit: 1000MS   Memory Limit: 65536K Total Submissions: 4914   Accepted: 1284   Special Judge Description The City has a number of municipal buildings and a number of fallout shelters that were build specially to hide municipal w…