Graph — Is the Graph Bipartite?

solve(graph) — graph[i] lists the neighbours of node i in an undirected
graph (nodes are 0..n-1). The graph may be disconnected.

Return true if the nodes can be split into two groups such that every edge
joins a node of one group to a node of the other — equivalently, if the graph can be
2-colored, or has no odd cycle.

Examples

solve([[1,3],[0,2],[1,3],[0,2]])        → true    // a 4-cycle
solve([[1,2,3],[0,2],[0,1,3],[0,2]])    → false   // contains a triangle
solve([[1,2],[0,2],[0,1]])              → false
solve([[]])                             → true

Remember to start a new search from every unvisited node: an odd cycle can hide
in a component the first search never reaches.