BZOJ4283: 魔法少女伊莉雅（最短路径图+最短路径树）

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题解

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题解

//minamoto#include<bits/stdc++.h>#define R register#define inf 0x3f3f3f3f#define pb push_back#define inline __inline__ __attribute__((always_inline))#define fp(i,a,b) for(R int i=(a),I=(b)+1;i<I;++i)#define fd(i,a,b) for(R int i=(a),I=(b)-1;i>I;--i)#define go(u) for(int i=head[u],v=e[i].v;i;i=e[i].nx,v=e[i].v)template<class T>inline bool cmin(T&a,const T&b){return a>b?a=b,1:0;}using namespace std;char buf[1<<21],*p1=buf,*p2=buf;inline char getc(){return p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<21,stdin),p1==p2)?EOF:*p1++;}int read(){R int res,f=1;R char ch;while((ch=getc())>'9'||ch<'0')(ch=='-')&&(f=-1);for(res=ch-'0';(ch=getc())>='0'&&ch<='9';res=res*10+ch-'0');return res*f;}char sr[1<<21],z[20];int C=-1,Z=0;inline void Ot(){fwrite(sr,1,C+1,stdout),C=-1;}void print(R int x){if(C>1<<20)Ot();if(x<0)sr[++C]='-',x=-x;while(z[++Z]=x%10+48,x/=10);while(sr[++C]=z[Z],--Z);sr[++C]='\n';}const int N=5e5+5,M=1e6+5;struct eg{int u,v,nx,w;}e[M];int head[N],tot;inline void add(R int u,R int v,R int w){e[++tot]={u,v,head[u],w},head[u]=tot;}struct node{int u,d;inline node(R int uu,R int dd):u(uu),d(dd){}inline bool operator <(const node &b)const{return d>b.d;}};priority_queue<node>q;int dis[2][N],fa[N];bool vis[N],flag[M];int n,m,res;void dij(int *dis,int S){memset(dis,0x3f,(n+1)<<2);memset(vis,0,(n+1)<<2);dis[S]=0,q.push(node(S,0));while(!q.empty()){int u=q.top().u;q.pop();if(vis[u])continue;vis[u]=1;go(u)if(cmin(dis[v],dis[u]+e[i].w))q.push(node(v,dis[v]));}}vector<int>g[N];void dfs(int u){for(int i=g[u].size()-1,v;~i;--i){v=g[u][i];dis[0][v]+dis[1][v]!=dis[0][n]?fa[v]=fa[u]:fa[v]=v;dfs(v);}}int main(){// freopen("testdata.in","r",stdin);n=read(),m=read(),res=inf;for(R int i=1,u,v,w;i<=m;++i)u=read(),v=read(),w=read(),add(u,v,w),add(v,u,w);dij(dis[0],1),dij(dis[1],n);fp(u,1,n)go(u)if(dis[1][u]==dis[1][v]+e[i].w)g[v].pb(u),flag[i]=1;fa[n]=n,dfs(n);for(R int i=1,u,v,w;i<=tot;++i){u=e[i].u,v=e[i].v,w=e[i].w;if(!flag[i]&&dis[0][u]<=dis[0][v]&&fa[u]!=fa[v]&&dis[0][u]+w+dis[1][v]!=dis[0][n])cmin(res,dis[0][u]+w+dis[1][v]);}printf("%d\n",res==inf?-1:res);return 0;}

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