#include<stdio.h>
#include<string.h>
#define M  10000000
#define min(a,b) (a)>(b)?(b):(a)
int map[202][202];
int vis[202],dist[202];
int n,s,t,flag;
int  dijkstra(int s)
{int min,i,j,k;memset(vis,0,sizeof(vis));for(i=0; i<n; i++)dist[i]=M;dist[s]=0;for(i=0; i<n; i++){k=-1;for(j=0; j<n; j++)if(!vis[j]&&(k==-1||dist[j]<dist[k]))k=j;if(dist[k]>7||k==-1){flag=0;return 0;}vis[k]=1;for(j=0; j<n; j++)dist[j]=min(dist[j],dist[k]+map[k][j]);}return 1;
}int main()
{int i,j,x,y,z,m;while(scanf("%d%d",&n,&m)!=EOF){flag=1;for(i=0; i<n; i++)for(j=0; j<n; j++)map[i][j]=map[j][i]=M;for(i=0; i<m; i++){scanf("%d%d",&x,&y);map[y][x]=1;map[x][y]=1;}for(i=0;i<n;i++)if(!dijkstra(i))//少了一个！

break; if(flag) printf("Yes\n"); else printf("No\n"); } return 0; }

再贴一个spfa算法写的代码。

#include<stdio.h>
#include<queue>
#include<string.h>
#include<algorithm>
using namespace std;
struct stu{int one,two,val,next;
};
stu edge[30000];
int vid[3000],vist[30000],head[30000],t,N,M,flag;
int spfa(int s)
{memset(vid,0,sizeof(vid));memset(vist,0x3f3f3f3f,sizeof(vist));queue<int> q;q.push(s);vid[s]=1;vist[s]=0;while(!q.empty()){int u=q.front();q.pop();vid[u]=0;for(int i=head[u];i!=-1;i=edge[i].next){int v=edge[i].two;if(vist[v]>vist[u]+edge[i].val){vist[v]=vist[u]+edge[i].val;if(!vid[v]){q.push(v);vid[v]=1;}}}}for(int i=0;i<N;++i)if(vist[i]>7){flag=0;return 0;}return 1;
}
void get(int u,int v,int w)
{stu E={u,v,w,head[u]};edge[t]=E;head[u]=t++;
}
int main()
{while(scanf("%d%d",&N,&M)!=EOF){t=0;flag=1;memset(head,-1,sizeof(head));int i,j,a,b,c;for(i=0;i<M;i++){scanf("%d%d",&a,&b);get(a,b,1);get(b,a,1);}for(i=0;i<N;i++)if(!spfa(i))break;if(flag)printf("Yes\n");else printf("No\n");}return 0;
}

再贴一个floyd（）算法写的程序

#include<stdio.h>
#include<string.h>
#define INL 0x3f3f3f3f
#define min(a,b)  (a)>(b)?(b):(a)//这个地方弄错了，弄成大于了。把这个改了一下就不超时了
int x[110][110];
int n,m;
void floyd()
{for(int k=0;k<n;k++)for(int i=0;i<n;i++)for(int j=0;j<n;j++) x[i][j]=min(x[i][j],x[i][k]+x[k][j]);
}
int main()
{int i,j,k,flag,a,b;while(scanf("%d%d",&n,&m)!=EOF){for(i=0;i<n;i++)//这个地方应该写成n，错写成m了<img alt="哭" src="http://static.blog.csdn.net/xheditor/xheditor_emot/default/cry.gif" />for(j=0;j<m;j++)x[i][j]=INL;for(i=0;i<m;i++){scanf("%d%d",&a,&b);x[a][b]=x[b][a]=1;}floyd();flag=0;for(i=0;i<n;i++){for(j=0;j<n;j++)if(x[i][j]>7||x[i][j]==INL){flag=1;break;}if(flag)break;}if(flag) printf("No\n");else  printf("Yes\n");}return 0;
}