Method find

code/data-structures/union_find.cpp:3–3

struct union_find {
  vi p; union_find(int n) : p(n, -1) { }
  int find(int x) { return p[x] < 0 ? x : p[x] = find(p[x]); }
  bool unite(int x, int y) {
    int xp = find(x), yp = find(y);
    if (xp == yp) return false;