Language: C++

#include<bits/stdc++.h>
#define rep(i,s,t) for(int i=s;i<=t;i++)
#define per(i,s,t) for(int i=s;i>=t;i--)
#define p_b push_back
#define N 100010

using namespace std;

int n,m;
int s1[N],s2[N],n1,n2;
bool vis[N];
vector<int> e[N];

void dfs(int u){
	vis[u]=1;
	for(int i=0;i<e[u].size();i++)
	if(!vis[e[u][i]]){
		dfs(e[u][i]);
		s1[++n1]=e[u][i];
		break;
	}
}

void dfs2(int u){
	vis[u]=1;
	for(int i=0;i<e[u].size();i++)
	if(!vis[e[u][i]]){
		dfs2(e[u][i]);
		s2[++n2]=e[u][i];
		break;
	}
}

int main(){
	scanf("%d%d",&n,&m); int x,y;
	rep(i,1,m){
		scanf("%d%d",&x,&y);
		e[x].p_b(y); e[y].p_b(x);
	}
	vis[1]=1;
	dfs(1);
	dfs2(1);
	//rep(i,1,n1) printf("%d ",s1[i]); puts("");
	//rep(i,1,n2) printf("%d ",s2[i]); puts("");
	
	printf("%d\n",n1+n2+1);
	rep(i,1,n1) printf("%d ",s1[i]);
	printf("1");
	per(i,n2,1) printf(" %d",s2[i]);
	puts("");
	return 0;
}#include<bits/stdc++.h>
#define rep(i,s,t) for(int i=s;i<=t;i++)
#define per(i,s,t) for(int i=s;i>=t;i--)
#define p_b push_back
#define N 100010

using namespace std;

int n,m;
int s1[N],s2[N],n1,n2;
bool vis[N];
vector<int> e[N];

void dfs(int u){
	vis[u]=1;
	for(int i=0;i<e[u].size();i++)
	if(!vis[e[u][i]]){
		dfs(e[u][i]);
		s1[++n1]=e[u][i];
		break;
	}
}

void dfs2(int u){
	vis[u]=1;
	for(int i=0;i<e[u].size();i++)
	if(!vis[e[u][i]]){
		dfs2(e[u][i]);
		s2[++n2]=e[u][i];
		break;
	}
}

int main(){
	scanf("%d%d",&n,&m); int x,y;
	rep(i,1,m){
		scanf("%d%d",&x,&y);
		e[x].p_b(y); e[y].p_b(x);
	}
	vis[1]=1;
	dfs(1);
	dfs2(1);
	//rep(i,1,n1) printf("%d ",s1[i]); puts("");
	//rep(i,1,n2) printf("%d ",s2[i]); puts("");
	
	printf("%d\n",n1+n2+1);
	rep(i,1,n1) printf("%d ",s1[i]);
	printf("1");
	per(i,n2,1) printf(" %d",s2[i]);
	puts("");
	return 0;
}

Compiler output:
a.cc:52:2: error: stray '#' in program
   52 | }#include<bits/stdc++.h>
      |  ^
a.cc:52:3: error: 'include' does not name a type
   52 | }#include<bits/stdc++.h>
      |   ^~~~~~~
a.cc:60:5: error: redefinition of 'int n'
   60 | int n,m;
      |     ^
a.cc:9:5: note: 'int n' previously declared here
    9 | int n,m;
      |     ^
a.cc:60:7: error: redefinition of 'int m'
   60 | int n,m;
      |       ^
a.cc:9:7: note: 'int m' previously declared here
    9 | int n,m;
      |       ^
a.cc:61:5: error: redefinition of 'int s1 [100010]'
   61 | int s1[N],s2[N],n1,n2;
      |     ^~
a.cc:10:5: note: 'int s1 [100010]' previously declared here
   10 | int s1[N],s2[N],n1,n2;
      |     ^~
a.cc:61:11: error: redefinition of 'int s2 [100010]'
   61 | int s1[N],s2[N],n1,n2;
      |           ^~
a.cc:10:11: note: 'int s2 [100010]' previously declared here
   10 | int s1[N],s2[N],n1,n2;
      |           ^~
a.cc:61:17: error: redefinition of 'int n1'
   61 | int s1[N],s2[N],n1,n2;
      |                 ^~
a.cc:10:17: note: 'int n1' previously declared here
   10 | int s1[N],s2[N],n1,n2;
      |                 ^~
a.cc:61:20: error: redefinition of 'int n2'
   61 | int s1[N],s2[N],n1,n2;
      |                    ^~
a.cc:10:20: note: 'int n2' previously declared here
   10 | int s1[N],s2[N],n1,n2;
      |                    ^~
a.cc:62:6: error: redefinition of 'bool vis [100010]'
   62 | bool vis[N];
      |      ^~~
a.cc:11:6: note: 'bool vis [100010]' previously declared here
   11 | bool vis[N];
      |      ^~~
a.cc:63:13: error: redefinition of 'std::vector<int> e [100010]'
   63 | vector<int> e[N];
      |             ^
a.cc:12:13: note: 'std::vector<int> e [100010]' previously declared here
   12 | vector<int> e[N];
      |             ^
a.cc:65:6: error: redefinition of 'void dfs(int)'
   65 | void dfs(int u){
      |      ^~~
a.cc:14:6: note: 'void dfs(int)' previously defined here
   14 | void dfs(int u){
      |      ^~~
a.cc:75:6: error: redefinition of 'void dfs2(int)'
   75 | void dfs2(int u){
      |      ^~~~
a.cc:24:6: note: 'void dfs2(int)' previously defined here
   24 | void dfs2(int u){
      |      ^~~~
a.cc:85:5: error: redefinition of 'int main()'
   85 | int main(){
      |     ^~~~
a.cc:34:5: note: 'int main()' previously defined here
   34 | int main(){
      |     ^~~~