UVA-11248 Frequency Hopping (max Flow + min cut)

Topic: To a network, ask if there is a stream just C. If it does not exist, is there an arc, so that the capacity of the arc can be changed to just C flow?

Problem Analysis: First find the maximum flow, if the maximum flow is not smaller than C, then there must be a flow just C. Otherwise, find the minimum cut set and then enumerate each arc to change its capacity to see if there is a stream that is just c.

The code is as follows:

# include<iostream># include<cstdio># include<cmath># include<string># include<vector ># include<list># include<set># include<map># include<queue># include<cstring># Include<algorithm>using namespace std;# define LL long long# define REP (i,s,n) for (int i=s;i<n;++i) # define CL (A, b) memset (A,b,sizeof (a)) # define CLL (A,b,n) Fill (a,a+n,b) const double inf=1e30;const int Inf=1<<30;const int maxn=    10005;struct edge{int fr,to;    LL CAP,FW;        Edge (int _fr,int _to,ll _cap,ll _fw): fr (_fr), to (_to), Cap (_CAP), FW (_FW) {} BOOL operator < (const Edge &a) const{        if (fr==a.fr) return to<a.to;    Return fr<a.fr; }};vector<edge>edges,tedges,ans;vector<int>g[2*maxn];vector<int>mincut;int N,E,cur[105],vis[ 105],D[105];    LL c,flow;int s,t;void init () {flow=0;    S=1,t=n;    Edges.clear (); REP (i,0,n+1) g[i].clear ();}    void Addedge (int u,int v,ll c) {Edges.push_back (Edge (u,v,c,0));Edges.push_back (Edge (v,u,0,0));    int m=edges.size ();    G[u].push_back (m-2); G[v].push_back (m-1);}    BOOL BFS () {CL (vis,0);    queue<int>q;    Vis[s]=1;    d[s]=0;    Q.push (s);        while (!q.empty ()) {int X=q.front (); Q.pop ();            REP (I,0,g[x].size ()) {Edge &e=edges[G[x][i]];                if (!VIS[E.TO]&AMP;&AMP;E.CAP&GT;E.FW) {vis[e.to]=1;                d[e.to]=d[x]+1;            Q.push (e.to); }}} return vis[t];} LL DFS (int x,ll a) {if (x==t| |    a==0) return A;    LL flow=0,f;        for (int &i=cur[x];i<g[x].size (); ++i) {Edge &e=edges[G[x][i]];            if (d[x]+1==d[e.to]&& (F=dfs (E.to,min (A,E.CAP-E.FW))) >0) {e.fw+=f;            Edges[g[x][i]^1].fw-=f;            Flow+=f;            A-=f;        if (a==0) break; }} return flow;}        BOOL Dinic () {while (BFS ()) {CL (cur,0);        Flow+=dfs (S,inf);    if (flow>=c) return true; } return false;} voidFindcut () {mincut.clear ();        for (int i=0;i<edges.size (); i+=2) {Edge &e=edges[i];    if (vis[e.fr]&&!vis[e.to]) mincut.push_back (i);    }}void Tofind () {findcut ();    Ans.clear ();    Tedges.clear ();    int len=edges.size ();    REP (I,0,len) tedges.push_back (Edges[i]);    LL F=flow;        REP (I,0,mincut.size ()) {edges[mincut[i]].cap+=c;        if (Dinic ()) Ans.push_back (Edges[mincut[i]);        Flow=f;        Edges.clear ();    REP (J,0,len) edges.push_back (Tedges[j]);    }}void solve (int CAs) {printf ("Case%d:", CAs); if (c==0| |    Dinic ()) {printf ("possible\n");        }else{Tofind ();        int len=ans.size ();            if (len>0) {printf ("possible option:");            Sort (Ans.begin (), Ans.end ()); REP (I,0,len) printf ("(%d,%d)%c", Ans[i].fr,ans[i].to, (i==len-1)? '        \ n ': ', ');    }else printf ("Not possible\n");    }}void read () {int A, B;    LL C; while (e--) {scanf ("%d%d%lld", &a,&b,&AMP;C);    Addedge (A,B,C);    }}int Main () {int cas=0; while (scanf ("%d%d%lld", &n,&e,&c), n| e|        C) {init ();        Read ();    Solve (++cas); } return 0;}

　　

UVA-11248 Frequency Hopping (max Flow + min cut)