建图还是要想一想的...写一下吧
首先根据有源汇可行流建图，正向附加边满流证明有可行流
然后在这个残量网络上删掉 $(t,s,oo)$ 这条边，跑 $s->t$ 最大流就是最大流，$t->s$ 最大流就是最小流
#include <bits/stdc++.h>
#define int long long
#define LL long long
#define rep(i, s, t) for (register int i = (s), i##end = (t); i <= i##end; ++i)
#define dwn(i, s, t) for (register int i = (s), i##end = (t); i >= i##end; --i)
using namespace std;
inline int read() {
int x = 0, f = 1;
char ch;
for (ch = getchar(); !isdigit(ch); ch = getchar())
if (ch == '-')
f = -f;
for (; isdigit(ch); ch = getchar()) x = 10 * x + ch - '0';
return x * f;
}
const int maxn = 500010;
struct Dinic {
#define oo 21474832333333LL
int n, m, s, t;
int cur[maxn], head[maxn], nx[maxn];
struct Edge {
int from, to, caps;
Edge() {}
Edge(int _1, int _2, int _3) : from(_1), to(_2), caps(_3) {}
} es[maxn];
void add(int u, int v, int w) {
es[m] = Edge(u, v, w);
nx[m] = head[u];
head[u] = m++;
es[m] = Edge(v, u, 0);
nx[m] = head[v];
head[v] = m++;
}
Dinic() {
m = 0;
memset(head, -1, sizeof(head));
}
queue<int> q;
int dis[maxn];
int BFS() {
for (int i = 0; i <= n; i++) dis[i] = 0;
dis[t] = 1;
q.push(t);
while (!q.empty()) {
int now = q.front();
q.pop();
for (int i = head[now]; i != -1; i = nx[i]) {
Edge &e = es[i ^ 1];
if (!dis[e.from] && e.caps) {
dis[e.from] = dis[now] + 1;
q.push(e.from);
}
}
}
return (dis[s] > 1);
}
int DFS(int u, int a) {
if (u == t || !a)
return a;
int flow = 0, f;
for (int &i = cur[u]; i != -1; i = nx[i]) {
Edge &e = es[i];
if (dis[e.to] == dis[u] - 1 && (f = DFS(e.to, min(e.caps, a)))) {
flow += f;
a -= f;
e.caps -= f;
es[i ^ 1].caps += f;
if (a == 0)
return flow;
}
}
return flow;
}
int MaxFlow(int _s, int _t) {
s = _s, t = _t;
int flow = 0, f;
while (BFS()) {
for (int i = 0; i <= n; i++) cur[i] = head[i];
while (f = DFS(s, oo)) flow += f;
}
return flow;
}
} sol;
int ind[maxn];
signed main() {
int n = read(), m = read(), s = read(), t = read(), SS = n + 1, TT = n + 2;
for (int i = 1; i <= m; i++) {
int u = read(), v = read(), b = read(), a = read();
sol.add(u, v, a - b); ind[v] += b; ind[u] -= b;
} sol.n = n + 5;
int totflow = 0;
rep(u, 1, n) {
if(ind[u] > 0) sol.add(SS, u, ind[u]), totflow += ind[u];
if(ind[u] < 0) sol.add(u, TT, -ind[u]);
}
sol.add(t, s, oo);
int rflow = sol.MaxFlow(SS, TT);
if(totflow > rflow) {
cout << "please go home to sleep" << endl;
return 0;
} sol.es[m - 1].caps = sol.es[m - 2].caps = 0;
cout << oo - sol.MaxFlow(t, s) << endl;
}
#include <bits/stdc++.h>
#define int long long
#define LL long long
#define rep(i, s, t) for (register int i = (s), i##end = (t); i <= i##end; ++i)
#define dwn(i, s, t) for (register int i = (s), i##end = (t); i >= i##end; --i)
using namespace std;
inline int read() {
int x = 0, f = 1;
char ch;
for (ch = getchar(); !isdigit(ch); ch = getchar())
if (ch == '-')
f = -f;
for (; isdigit(ch); ch = getchar()) x = 10 * x + ch - '0';
return x * f;
}
const int maxn = 200010;
struct Dinic {
#define oo 21474832333333LL
int n, m, s, t;
int cur[maxn], head[maxn], nx[maxn];
struct Edge {
int from, to, caps;
Edge() {}
Edge(int _1, int _2, int _3) : from(_1), to(_2), caps(_3) {}
} es[maxn];
void add(int u, int v, int w) {
es[m] = Edge(u, v, w);
nx[m] = head[u];
head[u] = m++;
es[m] = Edge(v, u, 0);
nx[m] = head[v];
head[v] = m++;
}
Dinic() {
m = 0;
memset(head, -1, sizeof(head));
}
queue<int> q;
int dis[maxn];
int BFS() {
for (int i = 0; i <= n; i++) dis[i] = 0;
dis[t] = 1;
q.push(t);
while (!q.empty()) {
int now = q.front();
q.pop();
for (int i = head[now]; i != -1; i = nx[i]) {
Edge &e = es[i ^ 1];
if (!dis[e.from] && e.caps) {
dis[e.from] = dis[now] + 1;
q.push(e.from);
}
}
}
return (dis[s] > 1);
}
int DFS(int u, int a) {
if (u == t || !a)
return a;
int flow = 0, f;
for (int &i = cur[u]; i != -1; i = nx[i]) {
Edge &e = es[i];
if (dis[e.to] == dis[u] - 1 && (f = DFS(e.to, min(e.caps, a)))) {
flow += f;
a -= f;
e.caps -= f;
es[i ^ 1].caps += f;
if (a == 0)
return flow;
}
}
return flow;
}
int MaxFlow(int _s, int _t) {
s = _s, t = _t;
int flow = 0, f;
while (BFS()) {
for (int i = 0; i <= n; i++) cur[i] = head[i];
while (f = DFS(s, oo)) flow += f;
}
return flow;
}
} sol;
int ind[maxn];
signed main() {
int n = read(), m = read(), s = read(), t = read(), SS = n + 1, TT = n + 2;
for (int i = 1; i <= m; i++) {
int u = read(), v = read(), b = read(), a = read();
sol.add(u, v, a - b);
ind[v] += b;
ind[u] -= b;
}
sol.n = n + 5;
int totflow = 0;
rep(u, 1, n) {
if (ind[u] > 0)
sol.add(SS, u, ind[u]), totflow += ind[u];
if (ind[u] < 0)
sol.add(u, TT, -ind[u]);
}
sol.add(t, s, oo);
int rflow = sol.MaxFlow(SS, TT);
if (totflow > rflow) {
cout << "please go home to sleep" << endl;
return 0;
}
sol.es[m - 1].caps = sol.es[m - 2].caps = 0;
cout << sol.MaxFlow(s, t) << endl;
}记住，删边的时候不是 $m$，是 $sol.m$
来源：oschina
链接：https://my.oschina.net/u/4312735/blog/3615752