P2207 上学路线route

正反两遍spfa，判断某条边是否是在最短路上，如果是的话就建边，跑最大流

  1 #include<cstdio>
  2 #include<queue>
  3 #include<cstring>
  4 #include<algorithm>
  5 using namespace std;
  6 #define maxn 250000
  7 #define inf 0x3f3f3f3f
  8 int n,m,src,dec,ans,cur[maxn],d[maxn],ansx,y,z,num,front2[maxn*2];
  9 int lev[maxn],front[maxn*2],head,tail,que[maxn*2],tot=1,dis[maxn],dis1[maxn];
 10 bool vis[maxn];
 11 struct node{
 12     int to,next,cap;
 13 }e[maxn*2];
 14
 15 struct Edge{
 16     int u,v,next,w,d;
 17 }edge[maxn*2],edge2[maxn*2];
 18
 19 inline void add(int u,int v,int w)
 20 {
 21     e[++tot].to=v; e[tot].next=front[u]; e[tot].cap=w; front[u]=tot;
 22     e[++tot].to=u; e[tot].next=front[v]; e[tot].cap=0; front[v]=tot;
 23 }
 24
 25 void ins(int u,int v,int w,int d)
 26 {
 27     edge[++num].v=v;
 28     edge[num].w=w;
 29     edge[num].u=u;
 30     edge[num].next=front2[u];
 31     edge[num].d=d;
 32     front2[u]=num;
 33 }
 34
 35 char ch;
 36 inline void read(int &now)
 37 {
 38     ch=getchar(); now=0;
 39     while(ch>‘9‘||ch<‘0‘) ch=getchar();
 40     while(ch>=‘0‘&&ch<=‘9‘) now=now*10+ch-‘0‘,ch=getchar();
 41 }
 42
 43 bool bfs()
 44 {
 45     for(int i=src;i<=dec;i++) lev[i]=-1,cur[i]=front[i];
 46     head=tail=0;
 47     que[tail++]=src; lev[src]=0;
 48     while(head<tail)
 49     {
 50         for(int i=front[que[head]];i;i=e[i].next)
 51             if(e[i].cap>0&&lev[e[i].to]==-1)
 52             {
 53                 lev[e[i].to]=lev[que[head]]+1;
 54                 que[tail++]=e[i].to;
 55                 if(e[i].to==dec) return true;
 56             }
 57         head++;
 58     }
 59     return false;
 60 }
 61
 62 void spfa(int s)
 63 {
 64     queue<int>q;
 65     for(int i=1;i<=n;i++) dis[i]=inf;
 66     memset(vis,false,sizeof(false));
 67     vis[s]=1; dis[s]=0;
 68     q.push(s);
 69     while(!q.empty())
 70     {
 71         int cur=q.front(); q.pop();
 72         for(int i=front2[cur];i;i=edge[i].next)
 73         {
 74             if(dis[edge[i].v]>dis[cur]+edge[i].w)
 75             {
 76                 dis[edge[i].v]=dis[cur]+edge[i].w;
 77                 if(!vis[edge[i].v])
 78                 {
 79                     vis[edge[i].v]=1;
 80                     q.push(edge[i].v);
 81                 }
 82             }
 83         }
 84         vis[cur]=false;
 85     }
 86 }
 87
 88 int dinic(int u,int flow)
 89 {
 90     if(u==dec) return flow;
 91     int res=0,delta;
 92     for(int i=cur[u];i;i=e[i].next)
 93     {
 94         if(e[i].cap>0&&lev[e[i].to]>lev[u])
 95         {
 96             delta=dinic(e[i].to,min(e[i].cap,flow-res));
 97             if(delta)
 98             {
 99                 e[i].cap-=delta; e[i^1].cap+=delta;
100                 res+=delta; if(res==flow) break;
101             }
102         }
103     }
104     if(res!=flow) lev[u]=-1;
105     return res;
106 }
107
108 int main()
109 {
110     read(n); read(m);
111     for(int i=1;i<=m;i++)
112     {
113         int u,v,w,d;
114         read(u); read(v); read(w); read(d);
115         edge2[i].u=u; edge2[i].v=v; edge2[i].w=w; edge2[i].d=d;
116         ins(u,v,w,d);
117         ins(v,u,w,d);
118     }
119     src=1; dec=n;
120     spfa(src);
121     printf("%d\n",dis[n]);
122     for(int i=1;i<=n;i++) dis1[i]=dis[i];
123     spfa(dec);
124     for(int i=1;i<=m;i++)
125     {
126         if(dis1[edge2[i].u]<dis1[edge2[i].v])
127         {
128             if(edge2[i].w+dis1[edge2[i].u]+dis[edge2[i].v]==dis1[n])
129             add(edge2[i].u,edge2[i].v,edge2[i].d);
130         }
131         else{
132             if(edge2[i].w+dis1[edge2[i].v]+dis[edge2[i].u]==dis1[n])
133             add(edge2[i].v,edge2[i].u,edge2[i].d);
134         }
135     }
136     while(bfs())
137     ans+=dinic(src,inf);
138     printf("%d\n",ans);
139     return 0;
140 }