Tyvj P2207 上学路线route

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

#include<cstdio>
#include<queue>
#include<cstring>
#include<algorithm>
using namespace std;
#define maxn 250000
#define inf 0x3f3f3f3f
int n,m,src,dec,ans,cur[maxn],d[maxn],ansx,y,z,num,front2[maxn*2];
int lev[maxn],front[maxn*2],head,tail,que[maxn*2],tot=1,dis[maxn],dis1[maxn];
bool vis[maxn]; 
struct node{
    int to,next,cap;
}e[maxn*2];

struct Edge{
    int u,v,next,w,d;
}edge[maxn*2],edge2[maxn*2];

inline void add(int u,int v,int w)
{
    e[++tot].to=v; e[tot].next=front[u]; e[tot].cap=w; front[u]=tot;
    e[++tot].to=u; e[tot].next=front[v]; e[tot].cap=0; front[v]=tot;
}

void ins(int u,int v,int w,int d)
{
    edge[++num].v=v;
    edge[num].w=w;
    edge[num].u=u;
    edge[num].next=front2[u];
    edge[num].d=d;
    front2[u]=num;
}

char ch;
inline void read(int &now)
{
    ch=getchar(); now=0;
    while(ch>'9'||ch<'0') ch=getchar();
    while(ch>='0'&&ch<='9') now=now*10+ch-'0',ch=getchar();
}

bool bfs()
{
    for(int i=src;i<=dec;i++) lev[i]=-1,cur[i]=front[i];
    head=tail=0; 
    que[tail++]=src; lev[src]=0;
    while(head<tail)
    {
        for(int i=front[que[head]];i;i=e[i].next)
            if(e[i].cap>0&&lev[e[i].to]==-1)
            {
                lev[e[i].to]=lev[que[head]]+1; 
                que[tail++]=e[i].to;
                if(e[i].to==dec) return true;
            }
        head++;
    }
    return false;
}

void spfa(int s)
{
    queue<int>q;
    for(int i=1;i<=n;i++) dis[i]=inf;
    memset(vis,false,sizeof(false));
    vis[s]=1; dis[s]=0;
    q.push(s);
    while(!q.empty())
    {
        int cur=q.front(); q.pop();
        for(int i=front2[cur];i;i=edge[i].next)
        {
            if(dis[edge[i].v]>dis[cur]+edge[i].w)
            {
                dis[edge[i].v]=dis[cur]+edge[i].w;
                if(!vis[edge[i].v])
                {
                    vis[edge[i].v]=1;
                    q.push(edge[i].v);
                }
            }
        }
        vis[cur]=false;
    }
}

int dinic(int u,int flow)
{
    if(u==dec) return flow;
    int res=0,delta;
    for(int i=cur[u];i;i=e[i].next)
    {
        if(e[i].cap>0&&lev[e[i].to]>lev[u])
        {
            delta=dinic(e[i].to,min(e[i].cap,flow-res));
            if(delta)
            {
                e[i].cap-=delta; e[i^1].cap+=delta;
                res+=delta; if(res==flow) break;
            }
        }
    }
    if(res!=flow) lev[u]=-1;
    return res;
}

int main()
{
    read(n); read(m);
    for(int i=1;i<=m;i++)
    {
        int u,v,w,d;
        read(u); read(v); read(w); read(d);
        edge2[i].u=u; edge2[i].v=v; edge2[i].w=w; edge2[i].d=d;
        ins(u,v,w,d);
        ins(v,u,w,d);
    }
    src=1; dec=n;
    spfa(src);
    printf("%d
",dis[n]);
    for(int i=1;i<=n;i++) dis1[i]=dis[i];
    spfa(dec);
    for(int i=1;i<=m;i++)
    {
        if(dis1[edge2[i].u]<dis1[edge2[i].v])
        {
            if(edge2[i].w+dis1[edge2[i].u]+dis[edge2[i].v]==dis1[n])
            add(edge2[i].u,edge2[i].v,edge2[i].d);
        }
        else{
            if(edge2[i].w+dis1[edge2[i].v]+dis[edge2[i].u]==dis1[n])
            add(edge2[i].v,edge2[i].u,edge2[i].d);
        }
    }
    while(bfs())
    ans+=dinic(src,inf);
    printf("%d
",ans);
    return 0;
}