# Split graph in two parts, such that most nodes have even number of edges

We have given graph of at most $200$ nodes, we want to split the given graph in two parts, such that the number of nodes with even number of edges is maximized, note that the edges that are between nodes of different parts are destroyed and not counted.

All possible splittings are allowed, even the one where all nodes are in one part. Each node should be in one part, we are required to print the nodes of one part.

This given graph has $5$ nodes, and the splitting made here is best because all nodes have even number of edges in the same part.

My idea was to do BFS over the nodes and when we find node with odd number of edges to put one node in the other part, but I can't come up with some strong idea.

• Are empty sets of nodes allowed? – xskxzr Aug 10 at 8:13
• I think brute force solutions like breadth-first search will be infeasible because in a graph with 200 nodes there are 2^199 possible (non-symmetric) groupings. – Auberon Aug 10 at 8:49
• Yes, empty set of nodes is allowed, but note that the other set will contain all of the nodes, I was thinking about some fast bfs not the one trying all possible splittings. – someone12321 Aug 10 at 9:34

Otherwise let $v$ be a node with odd degree , now modify the graph by inverting the neighbourhood of $v$ (If $a$ and $b$ are neighbours of $v$ , then if they have an edge , remove it otherwise add it) and also remove $v$ from the graph , for the new graph , split it into 2 parts $A$ and $B$ such that all nodes in $A$ and $B$ have even degree , add $v$ back to $A$ if $A$ has even neighbours of $v$ , else add it to $B$. This works because in $A$(Or $B$ if we added $v$ to $B$) all degrees are even except for neighbours of $v$ which become even after adding $v$