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[N],dis[N],vis[N],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;
}

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.

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

UVa10986_Sending email(最短路)(小白书图论专题)