RT，求助大佬，调了一晚上，救救蒟蒻吧。

#include <cstdio>
#include <cstring>
#include <iostream>
#include <algorithm>
#include <map>
using namespace std;
#define max_(a, b) a > b ? a : b
#define min_(a, b) a < b ? a : b
#define INF 0x3f3f3f3f


namespace IO {
char buf[1<<21], *p1 = buf, *p2 = buf, buf1[1<<21];
inline char gc () {return p1 == p2 && (p2 = (p1 = buf) + fread (buf, 1, 1 << 21, stdin), p1 == p2) ? EOF : *p1++;}

#ifndef ONLINE_JUDGE
#endif

template<class I>
inline void read(I &x) {
    x = 0; I f = 1; char c = gc();
    while(c < '0' || c > '9') {if(c == '-') f = -1; c = gc(); }
    while(c >= '0' && c <= '9') {x = x * 10 + c - '0'; c = gc(); }
    x *= f;
}

template<class I>
inline void write(I x) {
    if(x == 0) {putchar('0'); return;}
    I tmp = x > 0 ? x : -x;
    if(x < 0) putchar('-');
    int cnt = 0;
    while(tmp > 0) {
        buf1[cnt++] = tmp % 10 + '0';
        tmp /= 10;
    }
    while(cnt > 0) putchar(buf1[--cnt]);
}

#define in(x) read(x)
#define outn(x) write(x), putchar('\n')
#define out(x) write(x), putchar(' ')

} using namespace IO;


int n;
const int maxn = 1e5+1;

struct node{
    int to, next;
}e[maxn<<1];

int cnt, head[maxn];

inline void add(int u, int v) {
    e[++cnt] = (node){v, head[u]};
    head[u] = cnt;
}

int u, v, sum;
map<int,int>f;
int tot;
map<int,int>g;
int in[maxn];
int vis[maxn];
int st;
int ans[maxn], tnt;

bool dfs(int u, int now) {
    // out(g[u]), out("\n");
    if(now == tot) {
        out(g[u]);
        return true;
    }
    for(register int i(head[u]); i; i = e[i].next) {
        int v = e[i].to;
        // out(g[v]), out(" ");
        if(!vis[v]) {
            vis[v] = 1;
            if(dfs(v, now+1)) {
                out(g[u]);
                return true;
            }
            vis[v] = 0;
        }
    }
}

int main() {
    read(n);
    for(register int i(1); i <= n; ++i) {
        read(u), read(v);
        if(!f[u]) f[u] = ++tot, g[tot] = u;
        if(!f[v]) f[v] = ++tot, g[tot] = v;
        add(f[u], f[v]), add(f[v], f[u]);
        ++in[f[u]], ++in[f[v]];
    }
    st = 1;
    for(register int i(1); i <= tot; ++i) {
        if(in[i] == 1) st = i;
    }
    vis[st] = 1;
    dfs(st, 1);
    return 0;
}