POJ2387 - Til the Cows Come Home（Dijkstra）

Til the Cows Come Home
Time Limit: 1000MS Memory Limit: 65536K
Total Submissions: 48111 Accepted: 16379
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

Note
注意判断重边

#include<iostream>
#include<cstdio>
#include<string.h>
#include<algorithm>
using namespace std;

#define INF  0x3f3f3f3f
const int maxn=1005;
int graph[maxn][maxn];
int dist[maxn];
int last[maxn];
int visit[maxn];
int fin_cnt;

void init( int n){
  memset(visit,0,sizeof(visit));
  memset(last,-1,sizeof(last));
  visit[1]=1;
  dist[1]=0;
  fin_cnt=1;
  for(int i=2;i <=n;i++){
    dist[i]=graph[1][i];
    if(dist[i] != INF)
      last[i]=1;
  }
}

void dijkstra(int N){
    int Min,Min_idx;
    while( fin_cnt < N)
    {
      Min = INF;
      Min_idx = -1;
      for(int i= 2;i<= N;i++){
        if(visit[i] ) continue;
        if(  Min > dist[i])
          Min= dist[i],Min_idx =i;
      }
      if(Min ==INF) break;
      visit[Min_idx] =1;
      fin_cnt ++;

      for(int i=2;i<= N ; i++){
        if(visit[i]) continue;
        if(dist[Min_idx]+graph[Min_idx][i] < dist[i])
          dist[i]=dist[Min_idx] +graph[Min_idx][i],last[i]=Min_idx;
      }
    }
}

int main(){
    int m,n;

    while(cin>>m>>n){

    for(int i=1;i< maxn;i++)
      for(int j=1;j< maxn;j++)
        graph[i][j]=(i==j? 0:INF);

    int t1,t2,t3;
    for(int i=0;i< m; i++){
      cin>>t1>>t2>>t3;
      if(graph[t1][t2]==0 || t3 < graph[t1][t2]) //判断重边
      graph[t1][t2]=graph[t2][t1]=t3;
    }
    init(n);
    dijkstra(n);
    cout<<dist[n]<<endl;

    }
    return 0;
}