floyd算法，三重循环的顺序问题，不要写错了

最外层的循环应该是，中间节点的变量从1~n：

1     for (k=1;k<=n;k++)
2         for (i=1;i<=n;i++)
3             for (j=1;j<=n;j++)
4                  dist[i][j] = min(dist[i][j], dist[i][k]+dist[k][j]);

正确代码

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 #define LL long long
 4 #define ULL unsigned long long
 5 
 6 const LL mod_1=1e9+7;
 7 const LL mod_2=998244353;
 8 
 9 const double eps_1=1e-5;
10 const double eps_2=1e-10;
11 
12 const int maxn=1e3+10;
13 
14 LL dist[maxn][maxn];
15 
16 int main()
17 {
18     LL i,j,k,n,m,u,v,w;
19     cin>>n>>m;
20     for (i=1;i<=n;i++)
21         for (j=1;j<=n;j++)
22             dist[i][j] = 1e15;
23     for (i=1;i<=n;i++)
24         dist[i][i] = 0;
25     for (i=1;i<=m;i++)
26     {
27         cin>>u>>v>>w;
28         dist[u][v] = dist[v][u] = min(dist[u][v], w);
29     }
30     
31     for (k=1;k<=n;k++)
32         for (i=1;i<=n;i++)
33             for (j=1;j<=n;j++)
34                  dist[i][j] = min(dist[i][j], dist[i][k]+dist[k][j]);
35     //可以经过的点，k=1~n，逐渐递增
36 
37     for (i=1;i<=n;i++)
38     {
39         for (j=1;j<=n;j++)
40         {
41             cout<<dist[i][j];
42             if (j!=n)
43                 cout<<" ";
44             else
45                 cout<<endl;
46         }       
47     }
48 
49     return 0;
50 }
51 /*
52 3 2
53 1 2 1
54 2 3 2
55 */

错误代码

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 #define LL long long
 4 #define ULL unsigned long long
 5 
 6 const LL mod_1=1e9+7;
 7 const LL mod_2=998244353;
 8 
 9 const double eps_1=1e-5;
10 const double eps_2=1e-10;
11 
12 const int maxn=1e3+10;
13 
14 LL dist[maxn][maxn];
15 
16 int main()
17 {
18     LL i,j,k,n,m,u,v,w;
19     cin>>n>>m;
20     for (i=1;i<=n;i++)
21         for (j=1;j<=n;j++)
22             dist[i][j] = 1e15;
23     for (i=1;i<=n;i++)
24         dist[i][i] = 0;
25     for (i=1;i<=m;i++)
26     {
27         cin>>u>>v>>w;
28         dist[u][v] = dist[v][u] = min(dist[u][v], w);
29     }
30     
31     for (i=1;i<=n;i++)
32         for (j=1;j<=n;j++)
33             for (k=1;k<=n;k++)
34                  dist[i][j] = min(dist[i][j], dist[i][k]+dist[k][j]);
35     //dist[1][2] i=1,j=2 一开始被确定 1->k->2 但是这样的k不存在，于是dist[1][2] = 初始设置的最大值
36 
37     for (i=1;i<=n;i++)
38     {
39         for (j=1;j<=n;j++)
40         {
41             cout<<dist[i][j];
42             if (j!=n)
43                 cout<<" ";
44             else
45                 cout<<endl;
46         }       
47     }
48 
49     return 0;
50 }
51 /*
52 3 2
53 1 2 1
54 2 3 2
55 */

错误样例：

 input:

5 10
4 3 7
1 4 7
2 3 9
5 4 8
3 2 2
2 5 2
2 3 8
4 5 3
1 4 5
3 4 1

output:

0 8 6 5 8
8 0 2 3 2
6 2 0 1 4
5 3 1 0 3
8 2 4 3 0

制造样例：

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 #define LL long long
 4 #define ULL unsigned long long
 5 
 6 const LL mod_1=1e9+7;
 7 const LL mod_2=998244353;
 8 
 9 const double eps_1=1e-5;
10 const double eps_2=1e-10;
11 
12 const int maxn=1e3+10;
13 
14 int a[maxn];
15 
16 int main()
17 {
18     int n,m,v_max=10,i,j,k;
19     srand(time(NULL));
20     
21     //先创建一棵树
22     //crayon ?
23 
24     //n=5;
25     //m=10;
26 
27     n=4;
28     m=8;
29 
30     cout<<n<<' '<<m<<endl;
31 
32     for (i=1;i<=n;i++)
33         a[i]=i;
34     random_shuffle(a+1,a+n+1);
35 
36     for (i=2;i<=n;i++)
37     {
38         j=rand()%(i-1)+1;
39         
40         cout<<a[i]<<' '<<a[j]<<' '<<rand()%v_max+1<<endl;
41     }
42 
43     for (i=1;i<=m-n+1;i++)
44     {
45         j=rand()%n+1;
46         while (1)
47         {
48             k=rand()%n+1;
49             if (k!=j)
50                 break;
51         }
52         cout<<j<<' '<<k<<' '<<rand()%v_max+1<<endl;
53     }
54 
55     return 0;
56 }