C++ :

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <time.h>
using namespace std;
int path[2005],g[505][505],n,k;
int maxn,minx;//求出最大和最小的点
void dfs(int v)
{
    int i;
    for(i=minx;i<=maxn;i++){
        if(g[v][i]){
            g[v][i]--;
            g[i][v]--;
            dfs(i);
        }
    }
    path[k++]=v;
}
int main()
{
    int i;
    int a,b;
    while(cin>>n){
        memset(g,0,sizeof(g));
        memset(path,0,sizeof(path));
        minx=505;
        maxn=0;
        k=0;
        for(i=0;i<n;i++){
            cin>>a>>b;
            g[a][b]++;
            g[b][a]++;
            g[a][0]++;//a的度
            g[b][0]++;//b的度
            maxn=(a<b?b:a)<maxn?maxn:(a<b?b:a);
            minx=minx<(a<b?a:b)?minx:(a<b?a:b);
        }
        for(i=minx;i<=maxn;i++){
            if(g[i][0]%2){
                dfs(i);
                break;
            }
        }
        if(i==maxn+1){
            dfs(1);
        }
        for(int j=k-1;j>=0;j--)
             printf("%d\n",path[j]);
    }
    return 0;
}

Pascal :

var
	n,i,j,s,t,m:word;
	cir:boolean;
	con:array[1..500,1..500]of word;
	d:array[1..500]of word;
	path:array[1..1025]of word;
procedure search(s:word);
var
  i:word;
begin
  for i:=1 to m do
    if con[s,i]>0 then
    begin
      dec(con[s,i]);
			dec(con[i,s]);
      search(i);
    end;
	inc(j);
	path[j]:=s;
end;
begin
	readln(n);
	for i:=1 to n do
	begin
		readln(s,t);
		inc(con[s,t]);
		inc(con[t,s]);
		inc(d[s]);
		inc(d[t]);
		if s>m then m:=s;
		if t>m then m:=t;
	end;
	cir:=true;
	for i:=1 to m do
		if odd(d[i]) then
		begin
			s:=i;
			cir:=false;
			break;
		end;
	j:=0;
	if cir then search(1) else search(s);
	for i:=n+1 downto 1 do writeln(path[i]);
end.