$\newcommand{\O}{\mathrm{O}}$

無向グラフの重みつき大域最小容量カットを求めるアルゴリズム. 愚直に $n-1$ 回 $s,t$ 間最小カットを求めるよりも高速に解くことができ, 計算量は $\O (mn + n^2 \log n)$ である.
元論文は "Computing Edge-Connectivity in Multigraphs and Capacitated Graphs" [Nagamochi and Ibaraki 1992] や "A Simple Min-Cut Algorithm" [Stoer, Wagner 1997] など
(関数)
solve(): 大域最小容量カットの値を返し, またメンバ変数 $ans \_ set$ にそれを達成するカット(頂点集合) が格納される.
時間計算量: $\O (mn + n^2 \log n)$
template<typename _Key, typename _Tp> class FibonacciHeap
{
public:
using data_type = pair<_Key, _Tp>;
class node
{
public:
data_type _data;
unsigned short int _child;
bool _mark;
node *_par, *_prev, *_next, *_ch_last;
node(data_type&& data) : _data(move(data)), _child(0), _mark(false),
_par(nullptr), _prev(nullptr), _next(nullptr), _ch_last(nullptr){}
inline const _Key& get_key() const noexcept { return _data.first; }
void insert(node *cur){
if(_ch_last) insert_impl(cur, _ch_last);
else _ch_last = cur, _ch_last->_prev = _ch_last->_next = _ch_last;
++_child, cur->_par = this;
}
void erase(node *cur){
if(cur == cur->_prev){
_ch_last = nullptr;
}else{
erase_impl(cur);
if(cur == _ch_last) _ch_last = cur->_prev;
}
--_child, cur->_par = nullptr;
}
};
private:
size_t _size;
node *_minimum;
vector<node*> rank;
static void insert_impl(node *cur, node *next){
cur->_prev = next->_prev, cur->_next = next;
cur->_prev->_next = cur, next->_prev = cur;
}
static void erase_impl(node *cur){
cur->_prev->_next = cur->_next, cur->_next->_prev = cur->_prev;
}
void root_insert(node *cur){
if(_minimum){
insert_impl(cur, _minimum);
if(cur->get_key() < _minimum->get_key()) _minimum = cur;
}else{
_minimum = cur, _minimum->_prev = _minimum->_next = _minimum;
}
}
void root_erase(node *cur){
if(cur == cur->_prev) _minimum = nullptr;
else erase_impl(cur);
}
void _delete(node *cur){
root_erase(cur);
delete cur;
}
template<typename Key, typename Data>
node *_push(Key&& key, Data&& data){
++_size;
data_type new_data(forward<Key>(key), forward<Data>(data));
node* new_node = new node(move(new_data));
root_insert(new_node);
return new_node;
}
void _pop(){
assert(_size > 0);
--_size;
if(_size == 0){
_delete(_minimum);
return;
}
if(_minimum->_ch_last){
for(node *cur = _minimum->_ch_last->_next;;){
node *next = cur->_next;
_minimum->erase(cur), root_insert(cur);
if(!_minimum->_ch_last) break;
cur = next;
}
}
node *next_minimum = _minimum->_next;
for(node*& cur : rank) cur = nullptr;
for(node *cur = next_minimum; cur != _minimum;){
if(cur->get_key() < next_minimum->get_key()) next_minimum = cur;
node *next = cur->_next;
unsigned int deg = cur->_child;
if(rank.size() <= deg) rank.resize(deg + 1, nullptr);
while(rank[deg]){
if(cur->get_key() < rank[deg]->get_key() || cur == next_minimum){
root_erase(rank[deg]), cur->insert(rank[deg]);
}else{
root_erase(cur), rank[deg]->insert(cur);
cur = rank[deg];
}
rank[deg++] = nullptr;
if(rank.size() <= deg) rank.resize(deg + 1, nullptr);
}
rank[deg] = cur;
cur = next;
}
_delete(_minimum);
_minimum = next_minimum;
}
void _decrease_key(node *cur, const _Key& key){
assert(!(key < (_Key)0));
node *change = ((cur->_data.first -= key) < _minimum->get_key()) ? cur : nullptr;
if(!cur->_par || !(cur->get_key() < cur->_par->get_key())){
if(change) _minimum = change;
return;
}
while(true){
node *next = cur->_par;
next->erase(cur), root_insert(cur);
cur->_mark = false, cur = next;
if(!cur->_par) break;
if(!cur->_mark){
cur->_mark = true;
break;
}
}
if(change) _minimum = change;
}
void clear_dfs(node *cur){
if(cur->_ch_last){
for(node *_cur = cur->_ch_last->_next;;){
node *next = _cur->_next;
if(_cur == cur->_ch_last){
clear_dfs(_cur);
break;
}else{
clear_dfs(_cur);
}
_cur = next;
}
}
delete cur;
return;
}
void _clear(){
if(!_minimum) return;
for(node *cur = _minimum->_next;;){
node *next = cur->_next;
if(cur == _minimum){
clear_dfs(cur);
break;
}else{
clear_dfs(cur);
}
cur = next;
}
}
public:
FibonacciHeap() noexcept : _size(0u), _minimum(nullptr){}
// ~FibonacciHeap(){ _clear(); }
inline bool empty() const noexcept { return (_size == 0); }
inline size_t size() const noexcept { return _size; }
inline const data_type& top() const noexcept { return _minimum->_data; }
template<typename Key, typename Data>
node *push(Key&& key, Data&& data){ return _push(forward<Key>(key), forward<Data>(data)); }
void pop(){ _pop(); }
void decrease_key(node *cur, const _Key& key){ _decrease_key(cur, key); }
void clear(){ _clear(); _size = 0; rank.~vector<node*>(); }
};
class SimpleMergeSet
{
public:
vector<int> next;
SimpleMergeSet(const int node_size) : next(node_size){
iota(next.begin(), next.end(), 0);
}
void unite(const int u, const int v){
swap(next[u], next[v]);
}
};
template<typename T> class UndirectedMinCut
{
private:
struct edge{
int to; T cost; int rev;
edge(int _to, T _cost, int _rev)
: to(_to), cost(_cost), rev(_rev){}
};
const int V;
SimpleMergeSet ms;
vector<vector<edge> > G;
public:
vector<int> ans_set;
UndirectedMinCut(const int node_size) : V(node_size), ms(V), G(V){}
void add_edge(int u, int v, T cost){
G[u].emplace_back(v, cost, (int)G[v].size());
G[v].emplace_back(u, cost, (int)G[u].size() - 1);
}
T solve(){
T ans = numeric_limits<T>::max();
vector<typename FibonacciHeap<T, int>::node*> kp(V, nullptr);
vector<int> ord(V);
bool *visited = new bool[V]();
iota(ord.begin(), ord.end(), 0);
for(int i = V; i > 1; --i){
FibonacciHeap<T, int> fh;
vector<int> new_ord(i-1);
for(int id : ord){
kp[id] = fh.push(0, id);
}
for(int j = 0; j < i-1; ++j){
int cur = fh.top().second;
fh.pop();
visited[cur] = true, new_ord[j] = cur;
for(edge& e : G[cur]){
if(!visited[e.to]) fh.decrease_key(kp[e.to], e.cost);
}
}
int last_ver = fh.top().second, nx_last = new_ord[i-2];
for(edge& e : G[last_ver]){
if(e.to == nx_last) continue;
G[nx_last].emplace_back(e.to, e.cost, e.rev);
edge& reve = G[e.to][e.rev];
reve.to = nx_last, reve.rev = (int)G[nx_last].size() - 1;
}
for(int ver : new_ord){
visited[ver] = false;
}
visited[last_ver] = true;
if(ans > -fh.top().first){
ans = -fh.top().first;
ans_set.clear();
for(int cur = last_ver;;){
ans_set.push_back(cur);
if((cur = ms.next[cur]) == last_ver) break;
}
}
ms.unite(nx_last, last_ver);
swap(ord, new_ord);
}
delete[] visited;
return ans;
}
};