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 100000using 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;}

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 InputSample Output

32 1 0 10 1 1003 3 2 00 1 1000 2 2001 2 502 0 0 1

Case #1: 100Case #2: 150Case #3: unreachable

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Problemsetter: Igor Naverniouk