USACO Section 3.3: Riding the Fences

典型的找欧拉路径的题。先贴下USACO上找欧拉路径的法子：

• Pick a starting node and recurse on that node. At each step:
• If the node has no neighbors, then append the node to the circuit and return
• If the node has a neighbor, then make a list of the neighbors and process them (which includes deleting them from the list of nodes on which to work) until the node has no more neighbors
• To process a node, delete the edge between the current node and its neighbor, recurse on the neighbor, and postpend the current node to the circuit。

但实际写的时候其实用DFS起来更加方便。这道题我一开始用stack，但是始终不能找到输出的序列在500位制情况下最小的解，于是从网上看到有人用dfs轻松简单方便地就解决了。。。

 /*
 ID: yingzho1
 LANG: C++
 TASK: fence
 */
 #include <iostream>
 #include <fstream>
 #include <string>
 #include <map>
 #include <vector>
 #include <set>
 #include <algorithm>
 #include <stdio.h>
 #include <queue>
 #include <cstring>
 #include <cmath>
 #include <list>
 #include <cstdio>
 #include <cstdlib>
 #include <limits>
 #include <stack>

 using namespace std;

 ifstream fin("fence.in");
 ofstream fout("fence.out");

 ;

 };
 ];
 int F, st, ans_tot;

 void dfs(int st) {
     ; i < MAX; i++) {
         if (edge[st][i]) {
             edge[st][i]--, edge[i][st]--;
             du[st]--, du[i]--;
             dfs(i);
         }
     }
     ans[++ans_tot] = st;
 }

 int main()
 {
     fin >> F;
     int x, y;
     ; i < F; i++) {
         fin >> x >> y;
         edge[x][y]++, edge[y][x]++;
         du[x]++, du[y]++;
     }
     st = ;
     ; i < MAX; i++) {
         ) {
             st = i;
             break;
         }
     }
     ) {
         ; i < MAX; i++) {
             if (du[i]) {
                 st = i;
                 break;
             }
         }
     }
     dfs(st);
     ; i--) fout << ans[i] << endl;

     ;
 }