POJ　２３８７Til the Cows Come Home

Til the Cows Come Home

Time Limit:1000MS Memory Limit:65536KB 64bit IO Format:%I64d & %I64u

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Status

Practice

POJ 2387

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

90

题解：Dijdtra算法

代码：

#include<cstdio>
#include<string>
#include<cstring>
#include<cmath>
#include<iostream>
#include<algorithm>
using namespace std;
#define INF 10000001
int map[1001][1001];
int dis[1001];
bool vis[1001];
int n,m;

void dijkstra(int s)
{
int k;
for(int i=1; i<=n; i++)
dis[i]=map[s][i],vis[i]=false;
vis[s]=true;
dis[s]=0;
for(int i=1; i<=n; i++)
{
int MIN=INF;
for(int j=1; j<=n; j++)
if((!vis[j])&&(dis[j]<MIN))
{
k=j;
MIN=dis[j];
}

vis[k]=true;
for(int j=1; j<=n; j++)
if((!vis[j])&&(dis[k]+map[k][j]<dis[j]))
dis[j]=dis[k]+map[k][j];
}
}

void init()
{
for(int i=1; i<=n; i++)
for(int j=1; j<=n; j++)
{
if(i==j) map[i][j]=0;
else map[i][j]=INF;
}
}

int main()
{
int a,b,c;
scanf("%d%d",&m,&n);
init();
for(int i=0; i<m; i++)
{
scanf("%d%d%d",&a,&b,&c);
map[a][b]=map[b][a]=min(map[a][b],c);
}

dijkstra(1);
printf("%d\n",dis
);
return 0;
}