POJ 2230 Watchcow 【欧拉路】

Watchcow

Time Limit: 3000MS		Memory Limit: 65536K
Total Submissions: 6336		Accepted: 2743		Special Judge

Description

Bessie's been appointed the new watch-cow for the farm. Every night, it's her job to walk across the farm and make sure that no evildoers are doing any evil. She begins at the barn, makes her patrol, and then returns to the barn when she's done.

If she were a more observant cow, she might be able to just walk
each of M (1 <= M <= 50,000) bidirectional trails numbered 1..M
between N (2 <= N <= 10,000) fields numbered 1..N on the farm once
and be confident that she's seen everything she needs to see. But
since she isn't, she wants to make sure she walks down each trail
exactly twice. It's also important that her two trips along each trail
be in opposite directions, so that she doesn't miss the same thing
twice.

A pair of fields might be connected by more than one trail. Find a
path that Bessie can follow which will meet her requirements. Such a
path is guaranteed to exist.

Input

* Line 1: Two integers, N and M.

* Lines 2..M+1: Two integers denoting a pair of fields connected by a path.

Output

*
Lines 1..2M+1: A list of fields she passes through, one per line,
beginning and ending with the barn at field 1. If more than one solution
is possible, output any solution.

Sample Input

4 5
1 2
1 4
2 3
2 4
3 4

Sample Output

1
2
3
4
2
1
4
3
2
4
1

Hint

OUTPUT DETAILS:

Bessie starts at 1 (barn), goes to 2, then 3, etc...

Source

USACO 2005 January Silver

 

      题意分析：有一个农场看守，他每天晚上的工作就是巡视农场并且保证没有坏人破坏农场。 他从谷仓出发去巡视，并且最终会回到谷仓。

他需要每一条路走两遍，并且这两边必须是不同的方向，就是说：这次从u->v，下次就要从v->u。现在保证所给的数据一定存在满足题意的

路径。