CSU's 1256-day one-way street

Source: http://acm.csu.edu.cn/OnlineJudge/problem.php? Id = 1256

In a completely unidirectional directed graph, the minimum number of edges can be changed from 1 to n. Set the weight of the original edge to 0 and the weight of the edge in another direction to 1.

Resume list. Add_edge (u, v, 0); add_edge (v, U, 0); Use the spfa algorithm to find the minimum 1-> n weight, that is, the number of changes

Note that the maximum number of edges is 2*10000 rather than 10000, because this Te is really strange. It took a long time to find this error.

 1 #include<stdio.h> 2 #include<string.h> 3 #include<queue> 4 #define INF 0x3f3f3f3f 5 const int maxn=5000+5,maxm=20000+5; 6 int first[maxn],next[maxm],v[maxm],w[maxm],dist[maxn],numedge; 7 bool done[maxn]; 8 void add_edge(int a,int b,int c) 9 {10     v[numedge]=b;11     next[numedge]=first[a];12     first[a]=numedge;13     w[numedge]=c;14     numedge++;15 }16 void spfa(void)17 {18     std::queue<int>q;19     q.push(1);20     done[1]=true;21     while(!q.empty()){22         int a=q.front();23         q.pop();24         done[a]=false;25         for(int e=first[a];e!=-1;e=next[e]){26             if(dist[v[e]]>dist[a]+w[e]){27                 dist[v[e]]=dist[a]+w[e];28                 if(!done[v[e]]){29                     q.push(v[e]);30                     done[v[e]]=true;31                 }32             }33         }34     }35 }36 int main()37 {38     int n,m;39     while(scanf("%d%d",&n,&m)!=EOF){40         memset(first,-1,sizeof(int)*(n+1));41         numedge=1;42         int a,b;43         for(int e=1;e<=m;e++){44             scanf("%d%d",&a,&b);45             add_edge(a,b,0);46             add_edge(b,a,1);47         }48         memset(done,false,sizeof(bool)*(n+1));49         for(int i=1;i<=n;i++)50             dist[i]=(i==1?0:INF);51         spfa();52         if(dist[n]>=INF)53             printf("-1\n");54         else55             printf("%d\n",dist[n]);56     }57     return 0;58 }