最小费用最大流
最小费用最大流(Minimum Cost Maximum Flow,MCMF)。现在网络上的每条边,除了容量外,还有一个属性:单位费用。一条边上的费用等于流量×单位费用。我们知道,网络最大流往往可以用多种不同的方式达到,所以现在要求:在保持流最大的同时,找到总费用最少的一种。
https://www.luogu.com.cn/problem/P3381
只需将 EK 算法或 Dinic 算法中找增广路的过程,替换为用最短路算法寻找单位费用最小的增广路即可。
基于 Dinic 算法的实现:
#include<bits/stdc++.h>
//#define LOCAL
#define sum(a) ( accumulate ((a).begin(), (a).end(), 0int))
#define mine(a) (*min_element((a).begin(), (a).end()))
#define maxe(a) (*max_element((a).begin(), (a).end()))
#define mini(a) ( min_element((a).begin(), (a).end()) - (a).begin())
#define maxi(a) ( max_element((a).begin(), (a).end()) - (a).begin())
#define yes cout<<"YES"<<'\n'
#define no cout<<"NO"<<'\n'
#define print(x) cout<<(x)<<'\n'
#define print_v(x) for (int iii = 0; iii < (x).size() - 1; ++iii) {cout << (x)[iii] << ' ';}cout << (x)[(x).size() - 1]<< '\n'
using namespace std;
#define mp make_pair
#define int long long
const int N = 5e3 + 5, M = 1e5 + 5;
const int INF = 0x3f3f3f3f3f3f3f3f;
int n, m, tot = 1, lnk[N], cur[N], ter[M], nxt[M], cap[M], cost[M], dis[N], ret;
bool vis[N];
void add(int u, int v, int w, int c) {
ter[++tot] = v, nxt[tot] = lnk[u], lnk[u] = tot, cap[tot] = w, cost[tot] = c;
}
void addedge(int u, int v, int w, int c) { add(u, v, w, c), add(v, u, 0, -c); }
bool spfa(int s, int t) {
memset(dis, 0x3f, sizeof(dis));
memcpy(cur, lnk, sizeof(lnk));
queue<int> q;
q.push(s), dis[s] = 0, vis[s] = true;
while (!q.empty()) {
int u = q.front();
q.pop(), vis[u] = false;
for (int i = lnk[u]; i; i = nxt[i]) {
int v = ter[i];
if (cap[i] && dis[v] > dis[u] + cost[i]) {
dis[v] = dis[u] + cost[i];
if (!vis[v]) q.push(v), vis[v] = true;
}
}
}
return dis[t] != INF;
}
int dfs(int u, int t, int flow) {
if (u == t) return flow;
vis[u] = true;
int ans = 0;
for (int &i = cur[u]; i && ans < flow; i = nxt[i]) {
int v = ter[i];
if (!vis[v] && cap[i] && dis[v] == dis[u] + cost[i]) {
int x = dfs(v, t, min(cap[i], flow - ans));
if (x) ret += x * cost[i], cap[i] -= x, cap[i ^ 1] += x, ans += x;
}
}
vis[u] = false;
return ans;
}
int mcmf(int s, int t) {
int ans = 0;
while (spfa(s, t)) {
int x;
while ((x = dfs(s, t, INF))) ans += x;
}
return ans;
}
signed main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
#ifdef LOCAL
freopen("output.txt", "w", stdout);
freopen("input.txt", "r", stdin);
#endif
int s, t;
cin >> n >> m >> s >> t;
while (m--) {
int u, v, w, c;
cin >> u >> v >> w >> c;
addedge(u, v, w, c);
}
int ans = mcmf(s, t);
cout << ans << ' ' << ret;
}
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