luogu2865 路障 (dijkstra)

求次短路，dijkstra时同时记下到某点的最短距离和次短距离即可。

 #include<cstdio>
 #include<cstring>
 #include<algorithm>
 #include<vector>
 #include<queue>
 #include<map>
 #include<cmath>
 #include<ctime>
 #include<set>
 #define pa pair<int,int>
 #define lowb(x) ((x)&(-(x)))
 #define REP(i,n) for(i=1;i<=n;i++)
 #define MAX(a,b) ((a>b)?a:b)
 #define MIN(a,b) ((a<b)?a:b)
 #define CLR(a,x) memset(a,x,sizeof(a))
 #define rei register int
 using namespace std;
 const int maxn=,maxm=;
 typedef long long ll;

 ll rd(){
     ll x=;char c=getchar();int neg=;
     while(c<''||c>''){if(c=='-') neg=-;c=getchar();}
     while(c>=''&&c<='') x=x*+c-'',c=getchar();
     return x*neg;
 }

 struct Edge{
     int a,b,l,ne;
 }eg[maxm*];
 int M,N,egh[maxn],ect;
 int dis[maxn][];
 priority_queue<pa,vector<pa>,greater<pa> > q;

 inline void adeg(int a,int b,int l){
     eg[ect].a=a;eg[ect].b=b;eg[ect].l=l;
     eg[ect].ne=egh[a];egh[a]=ect++;
 }

 inline void change(int x,int d){
     if(d<dis[x][]){
         dis[x][]=dis[x][];dis[x][]=d;
         q.push(make_pair(d,x));
     }else if(d<dis[x][]&&d>dis[x][]){
         dis[x][]=d;q.push(make_pair(d,x));
     }
 }

 void dijkstra(){
     CLR(dis,);
     dis[][]=;q.push(make_pair(,));
     while(!q.empty()){
         int p=q.top().second;q.pop();
         for(int i=egh[p];i!=-;i=eg[i].ne){
             int b=eg[i].b;
             change(b,dis[p][]+eg[i].l);change(b,dis[p][]+eg[i].l);
         }
     }
 }

 int main(){
     //freopen(".in","r",stdin);
     rei i,j,k;
     N=rd(),M=rd();CLR(egh,-);
     REP(i,M){
         int a=rd(),b=rd(),c=rd();
         adeg(a,b,c);adeg(b,a,c);
     }//printf("lll\n");
     dijkstra();

     printf("%d\n",dis[N][]);
     return ;
 }