[luogu P1967][NOIp2013]P1967 货车运输

  1 #include<cstdio>
  2 #include<cstring>
  3 #include<iostream>
  4 #include<vector>
  5 #include<algorithm>
  6 #define inf 0x3f3f3f3f
  7 using namespace std;
  8 #define ss first
  9 #define tt second
 10 
 11 inline int read(){
 12     char ch;
 13     int re=0;
 14     bool flag=0;
 15     while((ch=getchar())!=‘-‘&&(ch<‘0‘||ch>‘9‘));
 16     ch==‘-‘?flag=1:re=ch-‘0‘;
 17     while((ch=getchar())>=‘0‘&&ch<=‘9‘)  re=re*10+ch-‘0‘;
 18     return flag?-re:re;
 19 }
 20 
 21 struct odge{
 22     int from,to,w;
 23     odge(int from=0,int to=0,int w=0):
 24         from(from),to(to),w(w){}
 25     bool operator < (const odge &n1) const {
 26         return w>n1.w;
 27     }
 28 };
 29 struct edge{
 30     int to,w,next;
 31     edge(int to=0,int w=0,int next=0):
 32         to(to),w(w),next(next){}
 33 };
 34 const int maxn=10001,maxm=50001;
 35 const int maxlogn=14;
 36 int n,m;
 37 vector<odge> odges;
 38 vector<edge> edges;
 39 int head[maxn];
 40 int par[maxn];
 41 int cnt=0;
 42 int F[maxn],son[maxn];
 43 bool vis[maxn];
 44 int sum,root;
 45 int depth[maxn],parent[maxlogn][maxn];
 46 int q;
 47 typedef pair<int,int> PII;
 48 vector<PII> ques;
 49 int length[maxn];
 50 
 51 inline void add_edge(odge e){
 52     edges.push_back(edge(e.to,e.w,head[e.from]));
 53     head[e.from]=++cnt;
 54     edges.push_back(edge(e.from,e.w,head[e.to]));
 55     head[e.to]=++cnt;
 56 }
 57 
 58 int find(int x){
 59     return par[x]==x?x:par[x]=find(par[x]);
 60 }
 61 
 62 void kruskal(){
 63     memset(head,0,sizeof head);
 64     edges.push_back(edge(0,0,0));
 65     int cnt1=1,cnt2=0;
 66     while(cnt2<m){
 67         odge e=odges[cnt2++];
 68         int dx=find(e.from),dy=find(e.to);
 69         if(dx==dy)  continue;
 70         add_edge(e);
 71         par[dy]=dx;
 72         if(++cnt1==n)  break;
 73     }
 74 }
 75 
 76 void getroot(int x,int fa){
 77     son[x]=1;  F[x]=0;
 78     for(int ee=head[x];ee;ee=edges[ee].next)
 79         if(edges[ee].to!=fa){
 80             getroot(edges[ee].to,x);
 81             son[x]+=son[edges[ee].to];
 82             F[x]=max(F[x],son[edges[ee].to]);
 83         }
 84     F[x]=max(F[x],sum-son[x]);
 85     if(F[x]<F[root])  root=x;
 86 }
 87 
 88 void dfs(int x,int fa,int deep){
 89     parent[0][x]=fa;
 90     depth[x]=deep;
 91     for(int ee=head[x];ee;ee=edges[ee].next)
 92         if(edges[ee].to!=fa){
 93             length[edges[ee].to]=edges[ee].w;
 94             dfs(edges[ee].to,x,deep+1);
 95         }
 96 }
 97 
 98 void init(){
 99     n=read();  m=read();
100     for(int i=0;i<m;i++){
101         int from=read(),to=read(),w=read();
102         odges.push_back(odge(from,to,w));
103     }
104     for(int i=1;i<=n;i++)
105         par[i]=i;
106     sort(odges.begin(),odges.end());
107     kruskal();
108     sum=F[root=0]=n;
109     getroot(1,0);
110     dfs(root,-1,0);
111     for(int k=0;k+1<maxlogn;k++)
112         for(int v=1;v<=n;v++)
113             if(parent[k][v]<0)  parent[k+1][v]=-1;
114             else  parent[k+1][v]=parent[k][parent[k][v]];
115     q=read();
116     for(int i=0;i<q;i++){
117         int from=read(),to=read();
118         ques.push_back(make_pair(from,to));
119     }
120 }
121 
122 int getlca(int u,int v){
123     if(depth[u]>depth[v])  swap(u,v);
124     for(int k=0;k<maxlogn;k++)
125         if(depth[v]-depth[u]>>k&1)  v=parent[k][v];
126     if(u==v)  return u;
127     for(int k=maxlogn-1;k>=0;k--)
128         if(parent[k][u]!=parent[k][v]){
129             u=parent[k][u];
130             v=parent[k][v];
131         }
132     return parent[0][u];
133 }
134 
135 void solve(){
136     for(int i=0;i<q;i++){
137         int u=ques[i].ss,v=ques[i].tt;
138         if(find(u)!=find(v)){
139             puts("-1");
140             continue;
141         }
142         int ans=inf;
143         int lca=getlca(ques[i].ss,ques[i].tt);
144         
145         while(u!=lca){
146             ans=min(ans,length[u]);
147             u=parent[0][u];
148         }
149         while(v!=lca){
150             ans=min(ans,length[v]);
151             v=parent[0][v];
152         }
153         
154         printf("%d\n",ans);
155     }
156 }
157 
158 int main(){
159     //freopen("temp.in","r",stdin);
160     init();
161     solve();
162     return 0;
163 }