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。
 1 #include<cstdio>
 2 #include<cstring>
 3 #include<algorithm>
 4 #include<vector>
 5 #include<queue>
 6 #include<map>
 7 #include<cmath>
 8 #include<ctime>
 9 #include<set>
10 #define pa pair<int,int>
11 #define lowb(x) ((x)&(-(x)))
12 #define REP(i,n) for(i=1;i<=n;i++)
13 #define MAX(a,b) ((a>b)?a:b)
14 #define MIN(a,b) ((a<b)?a:b)
15 #define CLR(a,x) memset(a,x,sizeof(a))
16 #define rei register int
17 using namespace std;
18 const int maxn=5050,maxm=100010;
19 typedef long long ll;
20 
21 ll rd(){
22     ll x=0;char c=getchar();int neg=1;
23     while(c<'0'||c>'9'){if(c=='-') neg=-1;c=getchar();}
24     while(c>='0'&&c<='9') x=x*10+c-'0',c=getchar();
25     return x*neg;
26 }
27 
28 struct Edge{
29     int a,b,l,ne;
30 }eg[maxm*2];
31 int M,N,egh[maxn],ect;
32 int dis[maxn][2];
33 priority_queue<pa,vector<pa>,greater<pa> > q;
34 
35 inline void adeg(int a,int b,int l){
36     eg[ect].a=a;eg[ect].b=b;eg[ect].l=l;
37     eg[ect].ne=egh[a];egh[a]=ect++;
38 }
39 
40 inline void change(int x,int d){
41     if(d<dis[x][0]){
42         dis[x][1]=dis[x][0];dis[x][0]=d;
43         q.push(make_pair(d,x));
44     }else if(d<dis[x][1]&&d>dis[x][0]){
45         dis[x][1]=d;q.push(make_pair(d,x));
46     }
47 }
48 
49 void dijkstra(){
50     CLR(dis,127);
51     dis[1][0]=0;q.push(make_pair(0,1));
52     while(!q.empty()){
53         int p=q.top().second;q.pop();
54         for(int i=egh[p];i!=-1;i=eg[i].ne){
55             int b=eg[i].b;
56             change(b,dis[p][0]+eg[i].l);change(b,dis[p][1]+eg[i].l);
57         }
58     }
59 }
60 
61 int main(){
62     //freopen(".in","r",stdin);
63     rei i,j,k;
64     N=rd(),M=rd();CLR(egh,-1);
65     REP(i,M){
66         int a=rd(),b=rd(),c=rd();
67         adeg(a,b,c);adeg(b,a,c);
68     }//printf("lll
");
69     dijkstra();
70     
71     printf("%d
",dis[N][1]);
72     return 0;
73 }