kuangbin专题四 J 题 POj1511（spfa）（还有一些疑惑）

kuangbin专题四 J 题 POj1511
Invitation Cards

Time Limit: 8000MS		Memory Limit: 262144K
Total Submissions: 29590		Accepted: 9888

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

题意：计算从源点到其他点来回的最短路之和

思路：数据较大，spfa正反各跑一次，累计源点到其他点之间的最短路

#include <iostream>
#include <cstring>
#include <queue>
#include <algorithm>
#include <cstdio>
using namespace std ;
//#define LL　long long
#define maxn 1001000
#define inf (1<<30)

int head[2][maxn] , n , m  ;
bool visit[maxn] ;
long long d[maxn] ;

struct node {
int e , w , next ;
};

node edge[2][maxn] ;

void spfa(int cap){
int v , i , b ;
queue<int>Q ;
for(int i=1 ; i<=n ; i++){
d[i] = inf ;
visit[i] = false ;
}

Q.push(1) ;
visit[1] = true ;
d[1] = 0 ;
while(!Q.empty()){
v = Q.front() ;
Q.pop() ;
visit[v] = false ;
for(int i = head[cap][v] ; i != -1 ; i = edge[cap][i].next){
// i 为 边 v->b 的 编号
// 松弛 边 v->b
b = edge[cap][i].e ;
if(d[b] > d[v] + edge[cap][i].w){
d[b] = d[v] + edge[cap][i].w ;
if(!visit[b]){
visit[b] = true ;
Q.push(b) ;
}
}
}
}
return;
}

int main(){
int t , a , b , c  ;
scanf("%d" , &t) ;
while(t--){

scanf("%d %d" , &n , &m) ;
// 边界
for(int i=1 ; i<=n ; i++){
head[0][i] = -1 ;
head[1][i] = -1 ;
}

for(int i=0 ; i<m ; i++){
scanf("%d %d %d" , &a , &b , &c) ;
edge[0][i].w = c ;
edge[0][i].e = b ;
edge[0][i].next = head[0][a] ;
head[0][a] = i ;

edge[1][i].w = c ;
edge[1][i].e = a ;
edge[1][i].next = head[1][b] ;
head[1][b] = i ;
}
long long ans = 0 ;
spfa(0) ;
for(int i=1 ; i<=n ; i++){
ans += d[i] ;
}
spfa(1) ;
for(int i=1 ; i<= n ; i++){
ans += d[i] ;
}
cout<<ans<<endl ;
}

return 0 ;
}