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unionfind.hpp
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unionfind.hpp
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#pragma once
struct UnionFind {
int n;
std::vector<int> parents;
UnionFind() {}
UnionFind(int n) : n(n), parents(n, -1) {}
int leader(int x) { return parents[x] < 0 ? x : parents[x] = leader(parents[x]); }
bool merge(int x, int y) {
x = leader(x), y = leader(y);
if (x == y) return false;
if (parents[x] > parents[y]) std::swap(x, y);
parents[x] += parents[y];
parents[y] = x;
return true;
}
bool same(int x, int y) { return leader(x) == leader(y); }
int size(int x) { return -parents[leader(x)]; }
std::vector<std::vector<int>> groups() {
std::vector<int> leader_buf(n), group_size(n);
for (int i = 0; i < n; i++) {
leader_buf[i] = leader(i);
group_size[leader_buf[i]]++;
}
std::vector<std::vector<int>> result(n);
for (int i = 0; i < n; i++) {
result[i].reserve(group_size[i]);
}
for (int i = 0; i < n; i++) {
result[leader_buf[i]].push_back(i);
}
result.erase(std::remove_if(result.begin(), result.end(), [&](const std::vector<int>& v) { return v.empty(); }), result.end());
return result;
}
void init(int n) { parents.assign(n, -1); } // reset
};