【luogu P3381 最小费用最大流】 模板

题目链接：https://www.luogu.org/problemnew/show/P3381

把bfs变成spfa

 #include <queue>
 #include <cstdio>
 #include <cstring>
 #include <iostream>
 #include <algorithm>
 using namespace std;
 const int maxn =  + ;
 int n, m, s, t, maxflow, mincost, dis[maxn], flow[maxn], pre[maxn], last[maxn];
 struct edge{
     int to, next, cost, flow;
 }e[maxn<<];
 int head[maxn], cnt = -;
 bool vis[maxn];
 queue<int> q;
 void add(int u, int v, int w, int c)
 {
     e[++cnt].next = head[u];
     e[cnt].to = v;
     e[cnt].cost = c;
     e[cnt].flow = w;
     head[u] = cnt;

     e[++cnt].next = head[v];
     e[cnt].to = u;
     e[cnt].cost = -c;
     e[cnt].flow = ;
     head[v] = cnt;
 }
 bool SPFA(int s, int t)
 {
     memset(dis, 0x7f, sizeof(dis));
     memset(flow, 0x7f, sizeof(flow));
     memset(vis, , sizeof(vis));
     q.push(s); pre[t] = -; vis[s] = ; dis[s] = ;
     while(!q.empty())
     {
         int now = q.front(); q.pop();
         vis[now] = ;
         for(int i = head[now]; i != -; i = e[i].next)
         {
             if(e[i].flow >  && dis[e[i].to] > dis[now] + e[i].cost)
             {
                 dis[e[i].to] = dis[now] + e[i].cost;
                 pre[e[i].to] = now;
                 last[e[i].to] = i;
                 flow[e[i].to] = min(flow[now], e[i].flow);
                 if(!vis[e[i].to])
                 {
                     q.push(e[i].to);
                     vis[e[i].to] = ;
                 }
             }
         }
     }
     return pre[t] != -;
 }
 void MCMF(int s, int t)
 {
     while(SPFA(s, t))
     {
         int now = t;
         maxflow += flow[t];
         mincost += flow[t] * dis[t];
         while(now != s)
         {
             e[last[now]].flow -= flow[t];
             e[last[now]^].flow += flow[t];
             now = pre[now];
         }
     }
 }
 int main()
 {
     memset(head, -, sizeof(head));
     scanf("%d%d%d%d",&n,&m,&s,&t);
     for(int i = ; i <= m; i++)
     {
         int u, v, w, c;
         scanf("%d%d%d%d",&u,&v,&w,&c);
         add(u, v, w, c);
     }
     MCMF(s, t);
     printf("%d %d\n",maxflow, mincost);
     return ;
 }