​​http://acm.scu.edu.cn/soj/problem.action?id=4444​​

先看 1与n之间是否有原图边 有的话就求补图最短路和a比较 没有就求原图最短路和b比较 都是一边BFS

#include <bits/stdc++.h>
using namespace std;
#define ll long long

struct node
{
    int v;
    int next;
};

queue <int> que0,que1,que2,que3,que4;
node edge[1000010];
ll dis[200010];
ll a,b;
int first[200010],book[200010],cnt[200010];
int n,m,num;

void addedge(int u,int v)
{
    edge[num].v=v;
    edge[num].next=first[u];
    first[u]=num++;
}

ll solveI()
{
    int i,u,v;
    while(!que0.empty()) que0.pop();
    memset(dis,0x3f,sizeof(dis));
    memset(book,0,sizeof(book));
    que0.push(1);
    dis[1]=0;
    book[1]=1;
    while(!que0.empty())
    {
        u=que0.front();
        que0.pop();
        for(i=first[u];i!=-1;i=edge[i].next)
        {
            v=edge[i].v;
            if(!book[v])
            {
                que0.push(v);
                dis[v]=dis[u]+a;
                book[v]=1;
            }
        }
    }
    return dis[n];
}

ll solveII()
{
    int i,u,v,sz,cur;
    while(!que1.empty()) que1.pop();
    while(!que2.empty()) que2.pop();
    while(!que3.empty()) que3.pop();
    while(!que4.empty()) que4.pop();
    memset(dis,0x3f,sizeof(dis));
    memset(book,0,sizeof(book));
    memset(cnt,0,sizeof(cnt));
    que1.push(1);
    for(i=2;i<=n;i++) que2.push(i);
    dis[1]=0;
    cur=1;
    while(!que1.empty()&&!que2.empty())
    {
        sz=que1.size();
        while(!que1.empty())
        {
            u=que1.front();
            que1.pop();
            for(i=first[u];i!=-1;i=edge[i].next)
            {
                v=edge[i].v;
                if(book[v]!=cur) cnt[v]=1;
                else cnt[v]++;
                book[v]=cur;
            }
        }
        while(!que2.empty())
        {
            u=que2.front();
            que2.pop();
            if(book[u]!=cur||cnt[u]<sz)
            {
                que3.push(u);
                dis[u]=cur*b;
            }
            else que4.push(u);
        }
        while(!que3.empty())
        {
            u=que3.front();
            que3.pop();
            que1.push(u);
        }
        while(!que4.empty())
        {
            u=que4.front();
            que4.pop();
            que2.push(u);
        }
        cur++;
    }
    return dis[n];
}

int main()
{
    int i,u,v,flag;
    while(scanf("%d%d%lld%lld",&n,&m,&a,&b)!=EOF)
    {
        memset(first,-1,sizeof(first));
        num=0,flag=0;
        for(i=1;i<=m;i++)
        {
            scanf("%d%d",&u,&v);
            addedge(u,v);
            addedge(v,u);
            if((u==1&&v==n)||(u==n&&v==1)) flag=1;
        }
        if(!flag) printf("%lld\n",min(b,solveI()));
        else printf("%lld\n",min(a,solveII()));
    }
    return 0;
}