P15961 [ICPC 2018 Jakarta R] Boomerangs

Let $G = (V, E)$ be a simple undirected graph with $N$ vertices and $M$ edges, where $V = \{1, \dots, N\}$. A tuple $\langle u, v, w \rangle$ is called as boomerang in $G$ if and only if $\{(u,v), (v,w)\} \subseteq E$ and $u \neq w$; in other words, a boomerang consists of two edges which share a common vertex. Given $G$, your task is to find as many disjoint boomerangs as possible in $G$. A set $S$ contains disjoint boomerangs if and only if each edge in $G$ only appears at most once (in one boomerang) in $S$. You may output any valid disjoint boomerangs, but the number of disjoint boomerangs should be maximum. For example, consider a graph $G = (V, E)$ of $N = 5$ vertices and $M = 7$ edges where $E = \{(1,2), (1,4), (2,3), (2,4), (2,5), (3,4), (3,5)\}$. :::align{center}  ::: The maximum number of disjoint boomerangs in this example graph is $3$. One example set containing the $3$ disjoint boomerangs is $\{\langle 4,1,2 \rangle, \langle 4,3,2 \rangle, \langle 2,5,3 \rangle\}$; no set can contain more than $3$ disjoint boomerangs in this example.

Input begins with a line containing two integers: $N$ $M$ ($1 \leq N, M \leq 100000$), representing the number of vertices and the number edges in $G$, respectively. The next $M$ lines, each contains two integers: $u_i$ $v_i$ ($1 \leq u_i < v_i \leq N$), representing the edge $(u_i,v_i)$ in $G$. You may safely assume that each edge appears at most once in the given list.

The first line of output contains an integer: $K$, representing the maximum number of disjoint boomerangs in $G$. The next $K$ lines, each contains three integers: $u$ $v$ $w$ (each separated by a single space), representing a boomerang $\langle u,v,w \rangle$. All boomerangs in the output should be disjoint. If there is more than one valid solution, you can output any of them.