BZOJ4424: Cf19E Fairy

树上差分的代码很简洁，dfs+差分即可

这题很多坑点啊，比如重边自环好坑

 #include<cstdio>
 #include<cstdlib>
 #include<algorithm>
 #include<cstring>
 #include<vector>
 #define pii pair<int,int>
 #define pb push_back
 #define mp make_pair
 #define ft first
 #define sc second
 #define MAXN 1000000+10
 using namespace std;
 int first[MAXN],edges[MAXN<<],nxt[MAXN<<],to[MAXN<<];
 int n,m,c=-;
 int b[MAXN],d[MAXN],tag[MAXN];
 int tmp,cnt,self_tmp;
 void insert(int x,int y,int e){
     nxt[++c]=first[x],first[x]=c,to[c]=y,edges[c]=e;
     nxt[++c]=first[y],first[y]=c,to[c]=x,edges[c]=e;
 }
 void init(){
     scanf("%d%d",&n,&m);
     int x,y;
     for(int i=;i<=m;i++){
         scanf("%d%d",&x,&y);
         if(x==y&&!self_tmp){self_tmp=i;continue;}
         if(x==y){self_tmp=-;continue;}
         insert(x,y,i);
     }
 }
 void dfs(int x,int fa){
     for(int e=first[x];e;e=nxt[e]){
         if(e==(fa^))continue;
         int &y=to[e];
         if(d[y]){
             if(d[x]<d[y])continue;
             if((d[x]-d[y]+)&){
                 tag[x]++,tag[y]--;
                 cnt++;tmp=edges[e];
             }
             else{
                 tag[x]--,tag[y]++;
             }
         }
         else{
             d[y]=d[x]+;
             dfs(y,e);
             tag[x]+=tag[y];
         }
     }
 }
 vector<int> ans;
 void find(int x){
     b[x]=;
     for(int e=first[x];e;e=nxt[e]){
         int &y=to[e];
         if(b[y])continue;
         if(tag[y]==cnt){
             ans.pb(edges[e]);
         }
         find(y);
     }
 }
 void solve(){
     if(-==self_tmp){printf("0\n");return;}
     for(int i=;i<=n;i++){
         if(!d[i])d[i]=,dfs(i,);
     }
     if(!cnt){
         if(self_tmp){printf("1\n%d\n",self_tmp);}
         else{printf("%d\n",m);for(int i=;i<m;i++)printf("%d ",i);printf("%d\n",m);}
         return;
     }
     if(self_tmp){printf("0\n");return;}
     for(int i=;i<=n;i++){
         if(!b[i])find(i);
     }
     if(cnt==)ans.pb(tmp);
     sort(ans.begin(),ans.end());
     printf("%d\n",ans.size());
     for(int i=;i<ans.size();i++){
         printf("%d",ans[i]);
         if(i==ans.size()-)printf("\n");
         else printf(" ");
     }
 }
 int main()
 {
 //  freopen("data.in","r",stdin);
 //  freopen("my.out","w",stdout);
     init();
     solve();
     return ;
 }