【2020牛客多校】2020牛客暑期多校训练营（第一场）H-Minimum-cost Flow——网络流
大致题意
给出一个费用流图，每条边的流量上限相同且不固定。
有\(q\)个询问，每个询问中给出每条边的流量上限（分数，且保证\(\leq 1\)）。当图中的流量为 \(1\) 个单位的时候，求出此时的费用。
分析
首先是询问个数，有\(1e5\)次询问，则需要预处理整个图，然后O(1)作答才可以过
然后注意到题目中给出的数据规模，图的边数只有\(100\)条
首先由于边的流量均为分数（\(\frac{u}{v}\)），而总流量为 \(1\) 个单位。我们先扩大\(\frac{v}{u}\)倍，将每条边的流量固定为 \(1\) 个单位，此时流量为 \(\frac{v}{u}\) 个单位
考虑在最大流使用 SPFA 查找路径时，当查找到一条路径时，此路径的流量一定为 \(1\) （根据上述的设定）。由于采用的本身便是最短路查找路径，得到的路径的费用为当前网络图下的最低费用。
所以可以稍微修改费用流板子中的一部分代码，使得每次得到路径并结算费用时，将每次得到的路径费用记录下来并保存。
#define ll long long
const int maxn = 100; //点数
const int INF = 0x3f3f3f3f;
struct Edge {
int from, to, cap, flow, cost;
Edge(int u, int v, int c, int f, int cc)
: from(u), to(v), cap(c), flow(f), cost(cc) {}
};
vector<ll> res;
struct MCMF {
int n, m;
vector<Edge> edges;
vector<int> G[maxn];
int inq[maxn]; //是否在队列中
int d[maxn]; //bellmanford
int p[maxn]; //上一条弧
int a[maxn]; //可改进量
void init(int n) {
this->n = n;
for (int i = 0; i <= n; ++i) G[i].clear();
edges.clear();
}
void addEdge(int from, int to, int cap, int cost) {
edges.emplace_back(Edge(from, to, cap, 0, cost));
edges.emplace_back(Edge(to, from, 0, 0, -cost));
m = int(edges.size());
G[from].emplace_back(m - 2);
G[to].emplace_back(m - 1);
}
bool spfa(int s, int t, int &flow, ll &cost) {
for (int i = 1; i <= n; ++i) d[i] = INF;
memset(inq, 0, sizeof(inq));
d[s] = 0;
inq[s] = 1;
p[s] = 0;
queue<int> q;
a[s] = INF;
q.push(s);
while (!q.empty()) {
int u = q.front();
q.pop();
inq[u] = 0;
for (int i = 0; i < int(G[u].size()); ++i) {
Edge &e = edges[G[u][i]];
if (e.cap > e.flow && d[e.to] > d[u] + e.cost) {
d[e.to] = d[u] + e.cost;
p[e.to] = G[u][i];
a[e.to] = min(a[u], e.cap - e.flow);
if (!inq[e.to]) {
q.push(e.to);
inq[e.to] = 1;
}
}
}
}
if (d[t] == INF) return false;
flow += a[t];
cost += (ll) d[t] * (ll) a[t];
res.push_back(d[t]);
for (int u = t; u != s; u = edges[p[u]].from) {
edges[p[u]].flow += a[t];
edges[p[u] ^ 1].flow -= a[t];
}
return true;
}
int MincostMaxflow(int s, int t, ll &cost) {
int flow = 0;
cost = 0;
while (spfa(s, t, flow, cost));
return flow;
}
} mcmf;
通过 res 数组记录下所有得到的路径的费用
而后分解流量。对于每一次询问，从 res 数组中从开始取值每次取出一条路径来提供 \(1\) 个单位的流量，直到满足流量要求，则停止取值并输出答案。
虽然 \(1 \leq u, v \leq 1e9\)，可能需要循环遍历 \(1e9\) 次才能出结果，但是由于图中每条边的容量都是 \(1\)，所以对于每一条路径，它只有被完全占用和完全空闲两种状态，而边数只有 \(100\) 条，即整个网络的最大流量只能是 \(100\) 个单位，即最多遍历 \(100\) 次，则复杂度不超过 \(100 * 1e5 = 1e7\)，当 \(MAX\_FLOW * u < v\) 时，则无解，输出 \(-1\)
注意一下乘法运算只需要考虑分子部分，分母部分不需要参与运算，在最后需要分子分母都除以 \(gcd\) 以保证最简
AC code
#include <bits/stdc++.h>
using namespace std;
#define ll long long
const int maxn = 100; //点数
const int INF = 0x3f3f3f3f;
struct Edge {
int from, to, cap, flow, cost;
Edge(int u, int v, int c, int f, int cc)
: from(u), to(v), cap(c), flow(f), cost(cc) {}
};
vector<long long> res;
struct MCMF {
int n, m;
vector<Edge> edges;
vector<int> G[maxn];
int inq[maxn]; //是否在队列中
int d[maxn]; //bellmanford
int p[maxn]; //上一条弧
int a[maxn]; //可改进量
void init(int n) {
this->n = n;
for (int i = 0; i <= n; ++i) G[i].clear();
edges.clear();
}
void addEdge(int from, int to, int cap, int cost) {
edges.emplace_back(Edge(from, to, cap, 0, cost));
edges.emplace_back(Edge(to, from, 0, 0, -cost));
m = int(edges.size());
G[from].emplace_back(m - 2);
G[to].emplace_back(m - 1);
}
bool spfa(int s, int t, int &flow, ll &cost) {
for (int i = 1; i <= n; ++i) d[i] = INF;
memset(inq, 0, sizeof(inq));
d[s] = 0;
inq[s] = 1;
p[s] = 0;
queue<int> q;
a[s] = INF;
q.push(s);
while (!q.empty()) {
int u = q.front();
q.pop();
inq[u] = 0;
for (int i = 0; i < int(G[u].size()); ++i) {
Edge &e = edges[G[u][i]];
if (e.cap > e.flow && d[e.to] > d[u] + e.cost) {
d[e.to] = d[u] + e.cost;
p[e.to] = G[u][i];
a[e.to] = min(a[u], e.cap - e.flow);
if (!inq[e.to]) {
q.push(e.to);
inq[e.to] = 1;
}
}
}
}
if (d[t] == INF) return false;
flow += a[t];
cost += (ll) d[t] * (ll) a[t];
res.push_back(d[t]);
for (int u = t; u != s; u = edges[p[u]].from) {
edges[p[u]].flow += a[t];
edges[p[u] ^ 1].flow -= a[t];
}
return true;
}
int MincostMaxflow(int s, int t, ll &cost) {
int flow = 0;
cost = 0;
while (spfa(s, t, flow, cost));
return flow;
}
} mcmf;
int n, m;
void solve() {
ios::sync_with_stdio(false);
cin.tie(0);
cout.precision(10);
cout << fixed;
while (cin >> n >> m) {
mcmf.init(n + 10);
res.clear();
for (int i = 0; i < m; ++i) {
int u, v, f;
cin >> u >> v >> f;
mcmf.addEdge(u, v, 1, f);
}
ll cost = 0;
ll mf = mcmf.MincostMaxflow(1, n, cost);
int q;
cin >> q;
while (q--) {
ll u, v;
cin >> u >> v;
if (mf * u < v) {
cout << "NaN" << '\n';
continue;
}
ll sum = 0;
ll ans = 0;
for (auto item : res) {
if (sum + u <= v) {
sum += u;
ans += item * u;
} else {
ans += (v - sum) * item;
break;
}
}
ll g = __gcd(ans, v);
cout << ans / g << '/' << v / g << '\n';
}
}
}
signed main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cout.tie(nullptr);
#ifdef ACM_LOCAL
freopen("in.txt", "r", stdin);
freopen("out.txt", "w", stdout);
int test_index_for_debug = 1;
char acm_local_for_debug;
while (cin >> acm_local_for_debug) {
if (acm_local_for_debug == '$') exit(0);
cin.putback(acm_local_for_debug);
if (test_index_for_debug > 20) {
throw runtime_error("Check the stdin!!!");
}
auto start_clock_for_debug = clock();
solve();
auto end_clock_for_debug = clock();
cout << "Test " << test_index_for_debug << " successful" << endl;
cerr << "Test " << test_index_for_debug++ << " Run Time: "
<< double(end_clock_for_debug - start_clock_for_debug) / CLOCKS_PER_SEC << "s" << endl;
cout << "--------------------------------------------------" << endl;
}
#else
solve();
#endif
return 0;
}
