UVa10986_Sending email(最短路)(小白书图论专题)

解题报告

思路：

裸裸的最短路。

#include <iostream>
#include <cstring>
#include <cstdio>
#include <queue>
#define inf 0x3f3f3f3f
#define N 40000
#define M 100000
using namespace std;
struct node
{
int v,w,next;
}edge[M];
int head
,dis
,vis
,cnt,n,m,s,t;
void add(int u,int v,int w)
{
edge[cnt].v=v;
edge[cnt].w=w;
edge[cnt].next=head[u];
head[u]=cnt++;
}
void spfa()
{
for(int i=0;i<n;i++)
{
dis[i]=inf;
vis[i]=0;
}
dis[s]=0;
vis[s]=1;
queue<int >Q;
Q.push(s);
while(!Q.empty()){
int u=Q.front();
Q.pop();
vis[u]=0;
for(int i=head[u];i!=-1;i=edge[i].next)
{
int v=edge[i].v;
if(dis[v]>dis[u]+edge[i].w)
{
dis[v]=dis[u]+edge[i].w;
if(!vis[v])
{
vis[v]=1;
Q.push(v);
}
}
}
}
}
int main()
{
int i,j,T,u,v,w,k=1;
scanf("%d",&T);
while(T--)
{
memset(head,-1,sizeof(head));
cnt=0;
scanf("%d%d%d%d",&n,&m,&s,&t);
for(i=0;i<m;i++)
{
scanf("%d%d%d",&u,&v,&w);
add(u,v,w);
add(v,u,w);
}
spfa();
printf("Case #%d: ",k++);
if(dis[t]==inf)
printf("unreachable\n");
else printf("%d\n",dis[t]);
}
return 0;
}

Problem E
Sending email
Time Limit: 3 seconds

"A new internet watchdog is creating a stir in Springfield. Mr. X, if that is his real name, has come up with a sensational scoop."

Kent Brockman
There are n SMTP servers connected by network cables. Each of the m cables connects two computers and has a certain latency measured in milliseconds required to
send an email message. What is the shortest time required to send a message from server S to server T along a sequence of cables? Assume that there is no delay incurred at any of the servers.
Input

The first line of input gives the number of cases, N. N test cases follow. Each one starts with a line containing n(2<=n<20000), m (0<=m<50000), S (0<=S<n)
and T (0<=T<n). S!=T. The next m lines will each contain 3 integers: 2 different servers (in the range [0, n-1]) that are connected by a
bidirectional cable and the latency, w, along this cable (0<=w<=10000).
Output

For each test case, output the line "Case #x:" followed by the number of milliseconds required to send a message from S toT. Print "unreachable" if there is no route from S to T.

Sample Input	Sample Output
3 2 1 0 1 0 1 100 3 3 2 0 0 1 100 0 2 200 1 2 50 2 0 0 1	Case #1: 100 Case #2: 150 Case #3: unreachable

Problemsetter: Igor Naverniouk