LCA || BZOJ 1602: [Usaco2008 Oct]牧场行走 || Luogu P2912 [USACO08OCT]牧场散步Pasture Walking

题面：[USACO08OCT]牧场散步Pasture Walking

题解：LCA模版题

代码：

 #include<cstdio>
 #include<cstring>
 #include<iostream>
 using namespace std;
 const int maxn=,max_log=;
 int N,Q,F[maxn+][max_log+],edge_head[maxn+],num_edge=,c,Dep[maxn+],u,v,w,A,B;
 int Sig[maxn+];
 struct Edge{
     int to,nx,dis;
 }edge[(maxn<<)+];
 inline void Add_edge(int from,int to,int dis){
     edge[++num_edge].nx=edge_head[from];
     edge[num_edge].to=to;
     edge[num_edge].dis=dis;
     edge_head[from]=num_edge;
     return;
 }
 inline void Dfs(int x,int fa){
     Dep[x]=Dep[fa]+;
     F[x][]=fa;
     for(int i=;i<=max_log;i++)
         F[x][i]=F[F[x][i-]][i-];
     for(int i=edge_head[x];i;i=edge[i].nx){
         int y=edge[i].to;
         if(y==fa)continue;
         Sig[y]=Sig[x]+edge[i].dis;
         Dfs(y,x);
     }
     return;
 }
 inline int LCA(int x,int y){
     if(Dep[x]<Dep[y]){
         int t=x;x=y;y=t;
     }
     for(int i=max_log;i>=;i--){
         int a;a=F[x][i];
         if(Dep[a]>=Dep[y])x=a;
         if(x==y)return x;
     }
     for(int i=max_log;i>=;i--){
         int a,b;a=F[x][i];b=F[y][i];
         if(a!=b){
             x=a;y=b;
         }
     }
     return F[x][];
 }
 int main(){
     scanf("%d%d",&N,&Q);
     for(int i=;i<N;i++){
         scanf("%d%d%d",&u,&v,&w);
         Add_edge(u,v,w);
         Add_edge(v,u,w);
     }
     Dfs(,);
     while(Q--){
         scanf("%d%d",&A,&B);
         c=LCA(A,B);
         printf("%d\n",Sig[A]+Sig[B]-Sig[c]-Sig[c]);
     }
     return ;
 }

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By:AlenaNuna