题目传送门：http://codeforces.com/contest/723/problem/E
官方题解：http://codeforces.com/blog/entry/47502
出度与入度相等 => 欧拉回路
有度数为偶数的点 => 两两配对之后加一条边
此时所有点的度数都为偶数，每一个连通子图存在欧拉回路
于是可以用随便定向，只有用网络流来的得出可行解
现在唯一的问题就是证明度数为奇数的点一定是偶数对了：
一条边贡献两个度 => 总度数为偶数
若度数为奇数的点有奇数个 => 总度数为奇数
矛盾 => 度数为奇数的点不可能有奇数个
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