Til the Cows Come Home（最短路径）

迈不过友情╰ 2022-08-04 13:42 111阅读 0赞

Til the Cows Come Home

<table> 
 <tbody> 
  <tr> 
   <td><strong>Time Limit:</strong>&nbsp;1000MS</td> 
   <td>&nbsp;</td> 
   <td><strong>Memory Limit:</strong>&nbsp;65536K</td> 
  </tr> 
  <tr> 
   <td><strong>Total Submissions:</strong>&nbsp;36299</td> 
   <td>&nbsp;</td> 
   <td><strong>Accepted:</strong>&nbsp;12352</td> 
  </tr> 
 </tbody> 
</table>

Description

Bessie is out in the field and wants to get back to the barn to get as much sleep as possible before Farmer John wakes her for the morning milking. Bessie needs her beauty sleep, so she wants to get back as quickly as possible.   
  
Farmer John's field has N (2 <= N <= 1000) landmarks in it, uniquely numbered 1..N. Landmark 1 is the barn; the apple tree grove in which Bessie stands all day is landmark N. Cows travel in the field using T (1 <= T <= 2000) bidirectional cow-trails of various lengths between the landmarks. Bessie is not confident of her navigation ability, so she always stays on a trail from its start to its end once she starts it.   
  
Given the trails between the landmarks, determine the minimum distance Bessie must walk to get back to the barn. It is guaranteed that some such route exists.

Input

\* Line 1: Two integers: T and N   
  
\* Lines 2..T+1: Each line describes a trail as three space-separated integers. The first two integers are the landmarks between which the trail travels. The third integer is the length of the trail, range 1..100.

Output

\* Line 1: A single integer, the minimum distance that Bessie must travel to get from landmark N to landmark 1.

Sample Input

5 5
    1 2 20
    2 3 30
    3 4 20
    4 5 20
    1 5 100

Sample Output

Hint

INPUT DETAILS:   
  
There are five landmarks.   
  
OUTPUT DETAILS:   
  
Bessie can get home by following trails 4, 3, 2, and 1.

Source

[USACO 2004 November][]

#include<cstdio>
    #include<cstring>
    #include<iostream>
    using namespace std;
    const int infinity=0x3f3f3f3f;
    const int maxnum=1005;
    int map1[maxnum][maxnum];//记录两个点的距离
    int vis[maxnum];//记录某一个点是否被访问！
    int dis[maxnum];//记录某一个点到第一个点的距离！
    int n;
    int djsta(int x,int y)
    {
        int i,j,mins,p;
        for(i=0;i<n;i++)
        {
            dis[i]=map1[x][i];//记录某一个点到第一个点的距离！
        }
        memset(vis,0,sizeof(vis));//默认都未被访问！
        vis[x]=1;
        for(i=0;i<n;i++)
        {
            mins=infinity;
            for(j=0;j<n;j++)
            {
                if(!vis[j]&&dis[j]<mins)
                {
                    p=j;
                    mins=dis[j];
                }
            }
            vis[p]=1;
            for(j=0;j<n;j++)
            {
                if(!vis[j]&&dis[p]+map1[p][j]<dis[j])
                {
                    dis[j]=dis[p]+map1[p][j];
                }
            }
        }
        return dis[y];
    }
    int main()
    {
        int t,i,j,a,b,c;
        while(cin>>t>>n)
        {
            for(i=0;i<=n;i++)
            {
                for(j=0;j<=n;j++)
                {
                    if(i==j)
                    {
                        map1[i][j]=map1[j][i]=0;
                    }
                    else
                    {
                        map1[i][j]=map1[j][i]=infinity;
                    }
                }
            }
            for(i=1;i<=t;i++)
            {
                cin>>a>>b>>c;
                if(map1[a][b]>c)
                {
                    map1[a][b]=map1[b][a]=c;
                }
            }
            cout<<djsta(n,1)<<endl;
        }
    }

[USACO 2004 November]: http://poj.org/searchproblem?field=source&key=USACO+2004+November