POJ 1511 Invitation Cards 链式前向星+spfa+反向建边

Invitation Cards

Time Limit: 8000MS		Memory Limit: 262144K
Total Submissions: 27200		Accepted: 9022

Description

In the age of television, not many people attend theater performances. Antique Comedians of Malidinesia are aware of this fact. They want to propagate theater and, most of all, Antique Comedies. They have printed invitation cards
with all the necessary information and with the programme. A lot of students were hired to distribute these invitations among the people. Each student volunteer has assigned exactly one bus stop and he or she stays there the whole day and gives invitation
to people travelling by bus. A special course was taken where students learned how to influence people and what is the difference between influencing and robbery.



The transport system is very special: all lines are unidirectional and connect exactly two stops. Buses leave the originating stop with passangers each half an hour. After reaching the destination stop they return empty to the originating stop, where they wait
until the next full half an hour, e.g. X:00 or X:30, where 'X' denotes the hour. The fee for transport between two stops is given by special tables and is payable on the spot. The lines are planned in such a way, that each round trip (i.e. a journey starting
and finishing at the same stop) passes through a Central Checkpoint Stop (CCS) where each passenger has to pass a thorough check including body scan.



All the ACM student members leave the CCS each morning. Each volunteer is to move to one predetermined stop to invite passengers. There are as many volunteers as stops. At the end of the day, all students travel back to CCS. You are to write a computer program
that helps ACM to minimize the amount of money to pay every day for the transport of their employees.

Input

The input consists of N cases. The first line of the input contains only positive integer N. Then follow the cases. Each case begins with a line containing exactly two integers P and Q, 1 <= P,Q <= 1000000. P is the number of stops
including CCS and Q the number of bus lines. Then there are Q lines, each describing one bus line. Each of the lines contains exactly three numbers - the originating stop, the destination stop and the price. The CCS is designated by number 1. Prices are positive
integers the sum of which is smaller than 1000000000. You can also assume it is always possible to get from any stop to any other stop.

Output

For each case, print one line containing the minimum amount of money to be paid each day by ACM for the travel costs of its volunteers.

Sample Input

2
2 2
1 2 13
2 1 33
4 6
1 2 10
2 1 60
1 3 20
3 4 10
2 4 5
4 1 50

Sample Output

46
210

思路：

已知有向边的关系，求解从1出发然后再重新回到1最关路径的总和。

线正向跑一边spfa。然后将输入的边反向建图在跑一次，两次累加的和就是结果。

数据较大，最后的累加和用long long。

反向建边在输入的时候与正向建边同时完成。开成一个二维的结构体编辑数组。第0维表示正向的，第1维表示反向的。

代码：

#include<iostream>
#include<string>
#include<algorithm>
#include<queue>
#include<cmath>
#include<cstring>
#include<cstdio>
#include<cstdlib>
using namespace std;
const int maxn=1000005;
const int maxm=2000005;
const int INF=0x3f3f3f3f;
struct edgenode {
    int to,next;
    long long w;
}edges[2][maxm];
int head[2][maxn],du[maxn];
long long dist[maxn];
bool vis[maxn];
int n,cnt;
bool spfa(int flag) {
    memset(vis,false,sizeof(vis));
    memset(du,0,sizeof(du));
    memset(dist,INF,sizeof(dist));
    queue<int> q;
    vis[1]=true;dist[1]=0;
    q.push(1);
    while(!q.empty()) {
        int now=q.front();q.pop();
        vis[now]=false;
        ++du[now];
        if(du[now]>n) return false;
        for(int i=head[flag][now];~i;i=edges[flag][i].next) {
            if(dist[edges[flag][i].to]>edges[flag][i].w+dist[now]) {
                dist[edges[flag][i].to]=dist[now]+edges[flag][i].w;
                if(!vis[edges[flag][i].to]) {
                    vis[edges[flag][i].to]=true;
                    q.push(edges[flag][i].to);
                }
            }
        }
    }
    return true;
}
void addedge(int u, int v, long long w, int flag) {
    edges[flag][cnt].to=v;
    edges[flag][cnt].w=w;
    edges[flag][cnt].next=head[flag][u];
    head[flag][u]=cnt;
}
int main() {
    int t,m;
    scanf("%d",&t);
    while(t--) {
        scanf("%d%d",&n,&m);
        int u,v;long long w,sum=0;
        cnt=0;
        for(int i=0;i<maxm;++i) {edges[0][i].next=-1;edges[1][i].next=-1;}
        for(int i=0;i<maxn;++i) {head[0][i]=-1;head[1][i]=-1;}
        for(int i=0;i<m;++i) {
            scanf("%d%d%lld",&u,&v,&w);
            addedge(u,v,w,0);
            addedge(v,u,w,1);
            ++cnt;
        }
        if(spfa(0)) for(int i=2;i<=n;++i) sum+=dist[i];
        if(spfa(1)) for(int i=2;i<=n;++i) sum+=dist[i];
        printf("%lld\n",sum);
    }
    return 0;
}