uva 10806

最小费用，最大流的思想，也不知道怎么稀里糊涂就ac啦

#include <iostream>
#include <cstring>
#include <queue>
using namespace std;
const int maxn=102;
const int inf=2<<20;
int cost[maxn][maxn],flow[maxn][maxn],cap[maxn][maxn],d[maxn];
int n,m;
int solve()
{
	queue<int> q;
	int i,vis[maxn],p[maxn],tot=0;
	while(1)
	{
		for(i=1;i<=n+1;i++)
		{
			d[i]=i==1?0:inf;
			vis[i]=0;
		}
		q.push(1);
		int x;
		while(!q.empty())
		{
			x=q.front();
			q.pop();
			vis[x]=0;
			for(i=1;i<=n+1;i++)
			{
				if(cap[x][i]>flow[x][i]&&d[i]>d[x]+cost[x][i])
				{
					d[i]=d[x]+cost[x][i];
					p[i]=x;
					if(!vis[i])
					{
						vis[i]=1;
						q.push(i);
					}
				}
			}
		}
		if(d[n+1]==inf) break;
		int min=inf;
		for(i=n+1;i!=1;i=p[i])
		{
			if(min>(cap[p[i]][i])-flow[p[i]][i])
				min=cap[p[i]][i]-flow[p[i]][i];
		}
		for(i=n+1;i!=1;i=p[i])
		{
			flow[p[i]][i]+=min;
			flow[i][p[i]]-=min;cost[i][p[i]]=-cost[i][p[i]];
		}
		tot+=min*d[n+1];
		if(flow[n][n+1]==2) return tot;
	}
	return -1;
}
int main()
{
	while(cin>>n&&n!=0)
	{
		cin>>m;
		int i;
		int tu,tv,tw;
		memset(flow,0,sizeof(flow));
		memset(cost,inf,sizeof(cost));
		memset(cap,0,sizeof(cap));
		for(i=1;i<=m;i++)
		{
			cin>>tu>>tv>>tw;
			cost[tu][tv]=tw;cap[tu][tv]=1;
			cost[tv][tu]=tw;cap[tv][tu]=1;
		}
		cap[n][n+1]=2;cost[n][n+1]=cost[n+1][n]=0;
		int ou=solve();
		if(ou!=-1) cout<<ou<<endl;
		else cout<<"Back to jail"<<endl;
	}
	return 0;
}