[Bzoj1726][Usaco2006 Nov]Roadblocks第二短路

#include<iostream>#include<cstdio>#include<queue>using namespace std;struct edge{int to,next,v;}e[200001];int n,r,cnt,head[5001];bool inq[5001];long long dis[5001],dis2[5001];queue<int> q;void insert(int u,int v,int w){e[++cnt]=(edge){v,head[u],w};head[u]=cnt;}void spfa(){for(int i=1;i<=n;i++)dis[i]=dis2[i]=1e15;int now,i;q.push(1);inq[1]=1;dis[1]=0;while(!q.empty()){now=q.front();q.pop();i=head[now];while(i){if(dis[now]+e[i].v<dis[e[i].to]){dis2[e[i].to]=dis[e[i].to];dis[e[i].to]=dis[now]+e[i].v;if(!inq[e[i].to]){inq[e[i].to]=1;q.push(e[i].to);}}else if(dis[now]+e[i].v<dis2[e[i].to]&&dis[now]+e[i].v>dis[e[i].to]){dis2[e[i].to]=dis[now]+e[i].v;if(!inq[e[i].to]){inq[e[i].to]=1;q.push(e[i].to);}}if(dis2[now]+e[i].v<dis2[e[i].to]){dis2[e[i].to]=dis2[now]+e[i].v;if(!inq[e[i].to]){inq[e[i].to]=1;q.push(e[i].to);}}i=e[i].next;}inq[now]=0;}if(dis2[n]!=1e15)printf("%d",dis2[n]);else printf("-1");}inline int read(){    int x=0,f=1;char ch=getchar();    while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}    while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}    return x*f;}int main(){n=read();r=read();for(int i=1;i<=r;i++){int a=read(),b=read(),d=read();insert(a,b,d);insert(b,a,d);}spfa();return 0;}