最小费用流: uva 1658

int MincostFlow(int s, int t, int flow_limit, int& cost) {
int flow = 0; cost = 0;
while(flow < flow_limit && BellmanFord(s, t, flow_limit, flow, cost));
return flow;
}

int minflow(int s,int t,long long &cost){
int flow=0;cost=0;
while(BellmanFord(s,t,flow,cost));
return flow
}

#include<iostream>
#include<string>
#include<cstring>
#include<algorithm>
#include<cstdio>
#include<vector>
#include<queue>
#include<bits/stdc++.h>
using namespace std;
#define mem(a,b) memset(a,b,sizeof(a));
#define sf scanf
#define pf printf
#define LL long long
const int maxn = 2000 + 10;
const int INF = 1000000000;
struct Edge {
int from, to, cap, flow, cost;
Edge(int u, int v, int c, int f, int w):from(u),to(v),cap(c),flow(f),cost(w) {}
};
struct MCMF {
int n, m;
vector<Edge> edges;
vector<int> G[maxn];
int inq[maxn];         // 是否在队列中
int d[maxn];           // Bellman-Ford
int p[maxn];           // 上一条弧
int a[maxn];           // 可改进量
void init(int n) {
this->n = n;
for(int i = 0; i < n; i++) G[i].clear();
edges.clear();
}
void addedge(int from, int to, int cap, int cost) {
edges.push_back(Edge(from, to, cap, 0, cost));
edges.push_back(Edge(to, from, 0, 0, -cost));
m = edges.size();
G[from].push_back(m-2);
G[to].push_back(m-1);
}
bool BellmanFord(int s, int t, int flow_limit, int& flow, int& cost) {
for(int i = 0; i < n; i++) d[i] = INF;
memset(inq, 0, sizeof(inq));
d[s] = 0; inq[s] = 1; p[s] = 0; a[s] = INF;
queue<int> Q;
Q.push(s);
while(!Q.empty()) {
int u = Q.front(); Q.pop();
inq[u] = 0;
for(int i = 0; i < G[u].size(); i++) {
Edge& e = edges[G[u][i]];
if(e.cap > e.flow && d[e.to] > d[u] + e.cost) {
d[e.to] = d[u] + e.cost;
p[e.to] = G[u][i];
a[e.to] = min(a[u], e.cap - e.flow);
if(!inq[e.to]) { Q.push(e.to); inq[e.to] = 1; }
}
}
}
if(d[t] == INF) return false;
if(flow + a[t] > flow_limit) a[t] = flow_limit - flow;
flow += a[t];
cost += d[t] * a[t];
for(int u = t; u != s; u = edges[p[u]].from) {
edges[p[u]].flow += a[t];
edges[p[u]^1].flow -= a[t];
}
return true;
}
// 需要保证初始网络中没有负权圈
int MincostFlow(int s, int t, int flow_limit, int& cost) {
int flow = 0; cost = 0;
while(flow < flow_limit && BellmanFord(s, t, flow_limit, flow, cost));
return flow;//返回的流量，但其实是没有用的，求的最小费用在&cost中求了
}
};
MCMF g;
int main(){
int n,m;
while(~sf("%d%d",&n,&m)){
g.init(n*2-2);

for(int i=2;i<=n-1;++i){
g.addedge(i-1,i+n-2,1,0);
}
// 点2~n-1拆成弧i->i'，前者编号为1~n-2，后者编号为n~2n-2
for(int i=1;i<=m;++i){
int u,v,c;
sf("%d%d%d",&u,&v,&c);
if(u!=1&&u!=n)u+=n-2;
else u--;
v--;
g.addedge(u,v,1,c);
}
int cost;
g.MincostFlow(0,n-1,2,cost);//s为0，  t为n-1
pf("%d\n",cost);

}

}