Graph — Valid Tree

solve(n, edges) — n nodes labelled 0..n-1 and a list of undirected
edges. Return true if the edges form a valid tree: connected, with no cycle.

Two facts make this short: a tree on n nodes has exactly n - 1 edges, and a
graph with n - 1 edges is a tree exactly when it has no cycle. Union–Find detects
a cycle the moment an edge joins two nodes that are already connected.

Examples

solve(5, [[0,1],[0,2],[0,3],[1,4]])          → true
solve(5, [[0,1],[1,2],[2,3],[1,3],[1,4]])    → false    // cycle 1-2-3
solve(4, [[0,1],[2,3]])                      → false    // two components
solve(1, [])                                 → true