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=5050,maxm=100010;
typedef long long ll;

ll rd(){
    ll x=0;char c=getchar();int neg=1;
    while(c<'0'||c>'9'){if(c=='-') neg=-1;c=getchar();}
    while(c>='0'&&c<='9') x=x*10+c-'0',c=getchar();
    return x*neg;
}

struct Edge{
    int a,b,l,ne;
}eg[maxm*2];
int M,N,egh[maxn],ect;
int dis[maxn][2];
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][0]){
        dis[x][1]=dis[x][0];dis[x][0]=d;
        q.push(make_pair(d,x));
    }else if(d<dis[x][1]&&d>dis[x][0]){
        dis[x][1]=d;q.push(make_pair(d,x));
    }
}

void dijkstra(){
    CLR(dis,127);
    dis[1][0]=0;q.push(make_pair(0,1));
    while(!q.empty()){
        int p=q.top().second;q.pop();
        for(int i=egh[p];i!=-1;i=eg[i].ne){
            int b=eg[i].b;
            change(b,dis[p][0]+eg[i].l);change(b,dis[p][1]+eg[i].l);
        }
    }
}

int main(){
    //freopen(".in","r",stdin);
    rei i,j,k;
    N=rd(),M=rd();CLR(egh,-1);
    REP(i,M){
        int a=rd(),b=rd(),c=rd();
        adeg(a,b,c);adeg(b,a,c);
    }//printf("lll
");
    dijkstra();
    
    printf("%d
",dis[N][1]);
    return 0;
}