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#10105. 「一本通 3.7 例 1」欧拉回路

求欧拉回路( dfs )

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#include <bits/stdc++.h>
using namespace std ;
 const int N=1e6+3,M=2e6+3;
  
  
  
 inline int read(){
    int res = 0, flag = 1;
    char ch = getchar();
    while (ch < '0' || ch > '9')
    {
        if (ch == '-')
            flag = -1;
        ch = getchar();
    }
    while (ch >= '0' && ch <= '9')
    {
        res = res * 10 + ch - '0';
        ch = getchar();
    }
    return res * flag;
}
 int n,m;
 // op==1 有向图 op==2 无向图
  
 vector<int> ans;
 int deg[N],in[N],out[N];
  
 int nxt[M],go[M],hd[N],all;
 int op;
 int vis[N];
 void add_(int x,int y){
    nxt[++all]=hd[x]; hd[x]=all;
    go[all]=y;
 }
  
 void dfs(int x){
    for(int &i=hd[x];i;i=nxt[i]){
        int y=go[i];
        int k=i;
         
        if(op==1){
            int e=(i+1)/2;
            if(vis[e]==0){
                vis[e]=1;
                dfs(y);
                 
                if(k&1) ans.push_back(e);
                else ans.push_back(-e);
            }
        }
        else{
            if(vis[k]==0){
                vis[k]=1;
                dfs(y);
                ans.push_back(k);
                 
            }
        }
    }
 }
 signed main(){
    int i,x,y;
    scanf("%d%d%d",&op,&n,&m);
     
    for(i=1;i<=m;i++){
        x=read(),y=read();
         
        if(op==1){
            add_(x,y),add_(y,x);
            deg[x]++,deg[y]++;
        }
        else{
            add_(x,y);
            out[x]++,in[y]++;
        }
    }
    if(op==1){
        for(i=1;i<=n;i++)
            if(deg[i]&1){ printf("NO\n"); return 0; }
    }
    else{
        for(i=1;i<=n;i++){
            if(in[i]!=out[i]){ printf("NO\n"); return 0; } 
        }
    }
     
    for(i=1;i<=n;i++){
        if(hd[i]){ dfs(i); break; }
    }
      
    if(ans.size()==m){
        cout<<"YES"<<endl;
        for(i=ans.size()-1;i>=0;i--)  printf("%d ",ans[i]);
    }
    else{
        cout<<"NO\n";
    }   
     
 }