POJ1847 Tram 最短路

Posted 2021-02-10 guaguastandup

这题用floyd和dijkstra都可以写，但是在用dijkstra时，如果将起始点的vis[a]标记为true，就会WA，但是在啊哈算法里面，是要提前标记的，我不知道为什么TUT，还在思索中

另外学到了一个新的表示方式

const int inf=0x3f3f3f3f;

memset(dis,0x3f,sizeof(dis));

 1 #include<iostream>
 2 #include<cstring>
 3 #include<algorithm>
 4 using namespace std;
 5 const int inf = 0x3f3f3f3f;
 6 const int N = 110;
 7 int e[N][N], dis[N];
 8 bool vis[N];
 9 int n;
10 void dijkstra(int a){
11     memset(dis, 0x3f, sizeof(dis));
12     dis[a] = 0;
13     //vis[a] = true;
14     for (int i = 1; i <= n-1;i++){
15         int t = 0;
16         int minn = inf;
17         for (int j = 1; j <= n;j++){
18             if(!vis[j]&&dis[j]<minn){
19                 minn = dis[j];
20                 t = j;
21             }
22         }
23         vis[t] = true;
24         for (int j = 1; j <= n;j++){
25                 dis[j] = min(dis[t] + e[t][j],dis[j]);
26         }
27     }
28 }
29 void floyd(){
30     int i, j, k;
31     for (k = 1; k <= n;k++)
32         for (i = 1; i <= n;i++)
33             for (j = 1; j <= n;j++)
34                 e[i][j] = min(e[i][k] + e[k][j],e[i][j]);
35 }
36 int main(){
37     int a, b;
38     cin >> n >> a >> b;
39     for (int i = 0; i <= n;i++){
40         for (int j = 0; j <= n;j++){
41             if(i==j)
42                 e[i][j] = 0;
43             else
44                 e[i][j] = inf;
45         }
46     }
47     for (int i = 1; i <= n;i++){
48         int m;
49         cin >> m;
50         for (int j = 1; j <= m;j++){
51             int x;
52             cin >> x;
53             if(j==1)
54                 e[i][x] = 0;
55             else
56                 e[i][x] = 1;
57         }
58     }
59 
60     dijkstra(a);
61     if(dis[b]==inf)
62         cout << -1 << endl;
63     else
64         cout << dis[b] << endl;
65 
66 
67     //方法2
68     floyd();
69     if(e[a][b]>=inf)
70         cout << -1 << endl;
71     else
72         cout << e[a][b] << endl;
73 
74     system("pause");
75     return 0;
76 }

 

pair和memset的头文件是cstring，在POJ里不能用万能头，所以要清楚头文件是什么