最小费用流模板(Bellman-Ford算法找最短路)

#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;
}