Uva 11248 Frequency Hopping

坑爹啊 c可能为0......又是两小时

先求出最大流 然后可能修改的边只能是最小割的边， 枚举没条边

保存修改之前的最大流 每次枚举时不重新找最大流直接在初始最大流上跑

只要最大流超过c就可以停止了

#include<cstdio>#include<cstring>#include<algorithm>#include<cmath>#include<vector>#include<queue>#include<stack>#include<iostream>using namespace std;typedef long long ll;const int MAXN = 107;const ll INF = 1LL << 51;struct Dinic {struct Edge {int from, to, flow;ll cap;Edge() {};Edge(int from, int to, ll cap, int flow):from(from), to(to), cap(cap), flow(flow) {}bool operator < (const Edge &e) const {return from < e.from || from == e.from && to < e.to;}};vector<Edge> edges, estmp;vector<int> G[MAXN];vector<int> cut;Edge able[MAXN*MAXN*2];int d[MAXN], cur[MAXN];int n, m, s, t;int nable;ll c, flow;bool vis[MAXN];void init(int n, ll c, int s, int t) {this->n = n, this->c = c, this->s = s, this->t = t;for(int i = 1; i <= n; i++) G[i].clear();edges.clear();flow = 0;}void addedge(int u, int v, ll cap) {edges.push_back( Edge(u, v, cap, 0));edges.push_back( Edge(v, u, 0, 0));m = edges.size();G[u].push_back(m - 2);G[v].push_back(m - 1);}bool bfs() {memset(vis, 0, sizeof(vis));memset(d, -1, sizeof(d));queue<int> q;q.push(s);d[s] = 0;vis[s] = true;while( !q.empty()) {int u = q.front(); q.pop();int i, sz = G[u].size();for(i = 0; i < sz; i++) {Edge e = edges[ G[u][i]];if( !vis[e.to] && e.cap > e.flow) {vis[e.to] = true;d[e.to] = d[u] + 1;q.push(e.to);}}}return vis[t];}ll dfs(int u, ll limit) {if(u == t || limit == 0) return limit;int flow = 0, f, sz = G[u].size();for(int &i = cur[u]; i < sz; i++) {Edge &e = edges[ G[u][i]];if(d[u] + 1 == d[e.to] && (f = dfs(e.to, min(limit, e.cap - e.flow) ) ) > 0) {e.flow += f;edges[ G[u][i] ^ 1].flow -= f;flow += f;limit -= f;if(limit == 0) break;}}return flow;}bool Maxflow() {while( bfs()) {memset(cur, 0, sizeof(cur));flow += dfs(s, INF);if(flow >= c) return true;}return false;}void getcut() {cut.clear();int i, sz = edges.size();for(i = 0; i < sz; i += 2) {if(vis[ edges[i].from] && !vis[ edges[i].to]) {cut.push_back(i);}}}void getable() {getcut();nable = 0;estmp.clear();int i, sz = edges.size();for(i = 0; i <sz; i++) estmp.push_back(edges[i]);ll lastflow = flow;sz = cut.size();for(i = 0; i < sz; i++) {edges[cut[i]].cap = edges[cut[i]].flow + c;if(Maxflow()) able[nable++] = edges[cut[i]];flow = lastflow;int j, csz = estmp.size();edges.clear();for(j = 0; j < csz; j++) edges.push_back(estmp[j]);}}}Dic;int main() {int n, e, id = 0;ll c, cap;while(~scanf("%d%d%lld", &n, &e, &c), n|e|c) {if(c < 0) while(1);Dic.init(n, c, 1, n);int i, u, v;for(i = 1; i <= e; i++) {scanf("%d%d%lld", &u, &v, &cap);Dic.addedge(u, v, cap);}printf("Case %d: ", ++id);if(Dic.Maxflow() || c == 0) puts("possible");else {Dic.getable();if(Dic.nable <= 0) puts("not possible");else {sort(Dic.able, Dic.able + Dic.nable);printf("possible option:(%d,%d)", Dic.able[0].from, Dic.able[0].to);for(i = 1; i < Dic.nable; i++)printf(",(%d,%d)", Dic.able[i].from, Dic.able[i].to);puts("");}}}return 0;}