What is Union-Find (Disjoint Set)? Connected components use case.

class UF {
  int[] p, r;
  int find(int x) { return p[x] == x ? x : (p[x] = find(p[x])); }
  void union(int a, int b) {
    a = find(a); b = find(b);
    if (a == b) return;
    if (r[a] < r[b]) p[a] = b; else { p[b] = a; if (r[a] == r[b]) r[a]++; }
  }
}