模板来自 《挑战程序设计竞赛》
poj 3068

#include <stdio.h>
#include <string.h>
#include <vector>
#include <algorithm>
using namespace std;

struct edge
{
    int to,cap,cost,rev;
    edge(int _to = 0, int _cap = 0, int _cost = 0, int _rev = 0)
    :to(_to),cap(_cap),cost(_cost),rev(_rev){}
};
const int INF = 0x3f3f3f3f;
const int MAX_V = 2222;
int V;
vector<edge> G[MAX_V];
int dist[MAX_V];
int prevv[MAX_V],preve[MAX_V];

void add_edge(int from, int to, int cap, int cost)
{
    G[from].push_back(edge(to,cap,cost,G[to].size()));
    G[to].push_back(edge(from,0,-cost,G[from].size()-1));
}

int min_cost_flow(int s, int t, int f)
{
    int res = 0;
    while(f > 0)
    {
        memset(dist,0x3f,sizeof(dist));
        dist[s] = 0;
        bool update = true;
        while(update)
        {
            update = false;
            for(int v = 0; v < V; ++v)
            {
                if(dist[v] == INF) continue;
                for(int i = 0; i < G[v].size(); ++i)
                {
                    edge& e = G[v][i];
                    if(e.cap > 0 && dist[e.to] > dist[v] + e.cost)
                    {
                        dist[e.to] = dist[v] + e.cost;
                        prevv[e.to] = v;
                        preve[e.to] = i;
                        update = true;
                    }
                }
            }
        }
        if(dist[t] == INF)
            return -1;
        int d = f;
        for(int v = t; v != s; v = prevv[v])
            d = min(d,G[prevv[v]][preve[v]].cap);
        f -= d;
        res += d*dist[t];
        for(int v = t; v != s; v = prevv[v])
        {
            edge& e = G[prevv[v]][preve[v]];
            e.cap -= d;
            G[v][e.rev].cap += d;
        }
    }
    return res;
}

int main()
{
    int n,m,u,v,cost,cnt = 0;
    while(scanf("%d %d",&n,&m) && n+m)
    {
        V = n;
        ++cnt;
        for(int i = 0; i <= n; ++i) G[i].clear();

        for(int i = 0; i < m; ++i)
        {
            scanf("%d %d %d",&u,&v,&cost);
            add_edge(u,v,1,cost);
        }
        int res = min_cost_flow(0,n-1,2);
        printf("Instance #%d: ",cnt);
        if(res == -1) printf("Not possible\n");
        else printf("%d\n",res);
    }
    return 0;
}