基本思想：
- 由于每条边只经过一次，遍历并删边即可.
#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
const int MAXN=2e5,MAXM=2e5;
struct Edge{
int from,to,nxt;
}e[MAXM];
int head[MAXN],edgeCnt=1;
void addEdge(int u,int v){
e[++edgeCnt].from=u;
e[edgeCnt].to=v;
e[edgeCnt].nxt=head[u];
head[u]=edgeCnt;
}
int stack[MAXN], ans[MAXN]; // 模拟系统栈，答案栈
bool vis[MAXN];
int top,t;
void euler() {
stack[++top] = 1;
while (top) {
int x = stack[top], i = head[x];
while (i && vis[i]) i = e[i].nxt;
if (i) {
stack[++top] = e[i].to;
head[x] = e[i].nxt;
vis[i]=vis[i^1]=1;
}else{
top--;
ans[++t] = x;
}
}
}
int main() {
int n,m;
scanf("%d%d",&n,&m);
for (int i = 1; i <= m; i++) {
int u,v;
scanf("%d%d",&u,&v);
addEdge(u,v);
addEdge(v,u);
}
euler();
for (int i = 1; i<=t; i++)
printf("%d
", ans[i]);
return 0;
}