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View the Project on GitHub knshnb/competitive_library
#include "src/Flow/FordFulkerson.hpp"
グラフの最大フローを求めるアルゴリズム。 最大流の流量をFとして、O(F|E|)。
増加パス(容量以内の正辺と、今までに流した流量を逆に押し戻す逆辺のみからなるパス)を1つ見つけて目一杯流すことを繰り返す。 dfsで増加パスを見つけるのがO(|E|)、これが最大O(F)回行われる可能性がある(実用上はこれよりかなり早く動作することが多い)。
/// @docs src/Flow/FordFulkerson.md template <class T = long long> struct FordFulkerson { struct Edge { int to, rev_idx; // 逆辺はg[to][rev_idx] T cap; bool is_rev; }; std::vector<std::vector<Edge>> g; FordFulkerson(int n) : g(n) {} void add_edge(int from, int to, T cap) { g[from].push_back({to, (int)g[to].size(), cap, false}); g[to].push_back({from, (int)g[from].size() - 1, 0, true}); } T max_flow(int s, int t) { std::vector<bool> used(g.size()); auto dfs = [this, &used, &t](auto f, int v, T min_acc) -> T { if (v == t) return min_acc; if (used[v]) return 0; used[v] = true; for (Edge& e : g[v]) { if (e.cap == 0) continue; T dif = f(f, e.to, std::min(min_acc, e.cap)); if (dif == 0) continue; e.cap -= dif, g[e.to][e.rev_idx].cap += dif; return dif; } return 0; }; T flow = 0; while (1) { std::fill(used.begin(), used.end(), false); T f = dfs(dfs, s, std::numeric_limits<T>::max() / 2); if (f == 0) return flow; flow += f; } } // max_flow()の後に呼ぶと、{u, v, 流した流量}のvectorを返す std::vector<std::tuple<int, int, T>> construct() { std::vector<std::tuple<int, int, T>> ret; for (int i = 0; i < g.size(); i++) { for (Edge& e : g[i]) { if (e.is_rev) continue; T f = g[e.to][e.rev_idx].cap; if (f == 0) continue; ret.push_back({i, e.to, f}); } } return ret; } };
#line 1 "src/Flow/FordFulkerson.hpp" /// @docs src/Flow/FordFulkerson.md template <class T = long long> struct FordFulkerson { struct Edge { int to, rev_idx; // 逆辺はg[to][rev_idx] T cap; bool is_rev; }; std::vector<std::vector<Edge>> g; FordFulkerson(int n) : g(n) {} void add_edge(int from, int to, T cap) { g[from].push_back({to, (int)g[to].size(), cap, false}); g[to].push_back({from, (int)g[from].size() - 1, 0, true}); } T max_flow(int s, int t) { std::vector<bool> used(g.size()); auto dfs = [this, &used, &t](auto f, int v, T min_acc) -> T { if (v == t) return min_acc; if (used[v]) return 0; used[v] = true; for (Edge& e : g[v]) { if (e.cap == 0) continue; T dif = f(f, e.to, std::min(min_acc, e.cap)); if (dif == 0) continue; e.cap -= dif, g[e.to][e.rev_idx].cap += dif; return dif; } return 0; }; T flow = 0; while (1) { std::fill(used.begin(), used.end(), false); T f = dfs(dfs, s, std::numeric_limits<T>::max() / 2); if (f == 0) return flow; flow += f; } } // max_flow()の後に呼ぶと、{u, v, 流した流量}のvectorを返す std::vector<std::tuple<int, int, T>> construct() { std::vector<std::tuple<int, int, T>> ret; for (int i = 0; i < g.size(); i++) { for (Edge& e : g[i]) { if (e.is_rev) continue; T f = g[e.to][e.rev_idx].cap; if (f == 0) continue; ret.push_back({i, e.to, f}); } } return ret; } };