Uva-11248-Frequency Hopping

大白书上面网络流的一道题，第一次接触这种保存残余网络流，然后直接在此基础之上进行更新，不过这个题我写的时候需要注意的地方就是最后找最小割后，进行不断枚举一个割边更新的时候一定要是原来的输入的边，而不是我们添加的反向边，在这里WA了1个多小时。

代码：

#include<cstdio>#include<cstring>#include<iostream>#include<queue>#include<algorithm>using namespace std;const int inf=2000000100;const int maxn=200;const int maxm=1e5;int n,m,c,st,des,cnt,e,head[maxn],nxt[maxm],pnt[maxm],from[maxm],flow[maxm],cap[maxm],level[maxn];bool vis[maxn];queue<int> q;struct Node{    int from;    int to;    Node(){};    Node(int sfrom,int sto)    {        from=sfrom;        to=sto;    }    bool operator < (const Node &a)const    {        if(from==a.from)            return to<a.to;        return from<a.from;    }}ans[maxm];void AddEdge(int u,int v,int f){    pnt[e]=v;from[e]=u;nxt[e]=head[u];cap[e]=f;head[u]=e++;    pnt[e]=u;from[e]=v;nxt[e]=head[v];cap[e]=0;head[v]=e++;}bool BFS(){    memset(level,0,sizeof(level));    level[st]=1;    q.push(st);    while(!q.empty())    {        int u=q.front();        q.pop();        for(int i=head[u];i!=-1;i=nxt[i])            if(cap[i]&&!level[pnt[i]])            {                level[pnt[i]]=level[u]+1;                q.push(pnt[i]);            }    }    return level[des];}bool bfs(){    memset(level,0,sizeof(level));    level[st]=1;    q.push(st);    while(!q.empty())    {        int u=q.front();        q.pop();        for(int i=head[u];i!=-1;i=nxt[i])            if(cap[i]>flow[i]&&!level[pnt[i]])            {                level[pnt[i]]=level[u]+1;                q.push(pnt[i]);            }    }    return level[des];}int DFS(int u,int maxf){    if(u==des||!maxf)        return maxf;    for(int i=head[u],t;i!=-1;i=nxt[i])        if(level[pnt[i]]==level[u]+1&&(t=DFS(pnt[i],min(maxf,cap[i]))))        {            cap[i]-=t;            cap[i^1]+=t;            return t;        }    return level[u]=0;}int dfs(int u,int maxf){    if(u==des||!maxf)        return maxf;    for(int i=head[u],t;i!=-1;i=nxt[i])        if(level[pnt[i]]==level[u]+1&&(t=dfs(pnt[i],min(maxf,cap[i]-flow[i]))))        {            flow[i]+=t;            flow[i^1]-=t;            return t;        }    return level[u]=0;}int maxflow(){    int ans=0;    while(BFS())        while(1)        {            int val=DFS(st,inf);            if(!val)                break;            ans+=val;        }    return ans;}int maxflow2(int flow){    int ans=0;    while(bfs())        while(1)        {            int val=dfs(st,inf);            if(!val)                break;            ans+=val;            if(flow+ans>=c)                return ans;        }    return ans;}void GetMinCut(int u){    vis[u]=1;    for(int i=head[u];i!=-1;i=nxt[i])        if(!vis[pnt[i]]&&cap[i])            GetMinCut(pnt[i]);}void ClearFlow(){    memset(flow,0,sizeof(flow));}void Solve(){    int flow=maxflow();    if(flow>=c)    {        printf("possible\n");        return;    }    cnt=0;    GetMinCut(st);    for(int i=0;i<e;i+=2)        if(vis[from[i]]&&!vis[pnt[i]])        {           // printf("Update %d %d %d\n",from[i],pnt[i],cap[i]);            cap[i]=c;            ClearFlow();            int val=maxflow2(flow);            if(val+flow>=c)                ans[cnt++]=Node(from[i],pnt[i]);               cap[i]=0;        }    if(!cnt)    {        printf("not possible\n");        return;    }    sort(ans,ans+cnt);    printf("possible option:");    printf("(%d,%d)",ans[0].from,ans[0].to);    for(int i=1;i<cnt;i++)        printf(",(%d,%d)",ans[i].from,ans[i].to);    printf("\n");}int main(){    int cas=1;    while(scanf("%d%d%d",&n,&m,&c)&&(n+m+c))    {        memset(head,-1,sizeof(head));        memset(vis,0,sizeof(vis));        e=0;        st=1,des=n;        for(int i=0;i<m;i++)        {            int u,v,f;            scanf("%d%d%d",&u,&v,&f);            AddEdge(u,v,f);        }        printf("Case %d: ",cas++);        if(n==1)        {            printf("possible\n");            continue;        }        Solve();    }    return 0;}