路障Roadblock
spfa版子（digstra更好2333）
1 #include<iostream> 2 #include<cstring> 3 #include<cstdio> 4 #define M 15000 5 #include<queue> 6 #define N 110 7 using namespace std; 8 int n,m; 9 int po,ans; 10 int head[N],to[M],next[M],len[M],e=1; 11 void buid(int u,int v,int l) 12 { 13 next[++e]=head[u],head[u]=e; 14 to[e]=v,len[e]=l; 15 } 16 int dis[N],init[N]; 17 int pre[N],fr[N],that[M],nu; 18 queue<int> q; 19 void spfa(int s) 20 { 21 memset(dis,20,sizeof(dis)); 22 dis[s]=0;init[s]=1,q.push(s); 23 while(!q.empty()) 24 { 25 int now=q.front();q.pop();init[now]=0; 26 for(int i=head[now];i;i=next[i]) 27 { 28 int j=to[i]; 29 if(dis[j]>dis[now]+len[i]) 30 { 31 dis[j]=dis[now]+len[i]; 32 pre[j]=i;fr[j]=now; 33 if(!init[j]) 34 { 35 init[j]=1;q.push(j); 36 } 37 } 38 } 39 } 40 } 41 int main() 42 { 43 scanf("%d%d",&n,&m); 44 for(int i=1;i<=m;++i) 45 { 46 int u,v,l; 47 scanf("%d%d%d",&u,&v,&l); 48 buid(u,v,l); 49 buid(v,u,l); 50 } 51 spfa(1);po=dis[n]; 52 int now=n; 53 while(now!=1) 54 { 55 that[++nu]=pre[now]; 56 now=fr[now]; 57 } 58 for(int i=1;i<=nu;++i) 59 { 60 len[that[i]]*=2; 61 len[that[i]^1]*=2; 62 spfa(1); 63 ans=max(ans,dis[n]); 64 len[that[i]]/=2; 65 len[that[i]^1]/=2; 66 } 67 cout<<ans-po<<endl;//end 68 return 0; 69 }
