hdu 2874 (LCA)

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虚拟赛的时候，一看是求任意两点的最短路，不管怎么优化都超时，最近做一些树的题目，这是求任意两点到最近公共祖先的距离，先用并差集判断是否联通，联通就求最近公共祖先，先把图建成一棵棵树，

#include<string.h>#include<stdio.h>#define N 10010int father[N],dfs[N],n,vis[N],head[N],num,f[N],ans,dis[N];struct edge{int st,ed,next,w;}E[N*2];void addedge(int x,int y,int w){E[num].st=x;E[num].ed=y;E[num].w=w;E[num].next=head[x];head[x]=num++;}int find(int a){if(f[a]!=a)f[a]=find(f[a]);return f[a];}void dfs1(int u){int i,v;vis[u]=1;for(i=head[u];i!=-1;i=E[i].next){v=E[i].ed;if(vis[v]==1)continue;father[v]=u;dfs[v]=dfs[u]+1;dis[v]=E[i].w;dfs1(v);}if(ans<dfs[u])ans=dfs[u];}int LCA(int x,int y){int sum=0;while(dfs[x]>dfs[y]){sum+=dis[x];x=father[x];}while(dfs[y]>dfs[x]){sum+=dis[y];y=father[y];}while(x!=y){sum+=(dis[x]+dis[y]);x=father[x];y=father[y];}return sum;}int main(){int i,j,k,x,y,m,ans,w;while(scanf("%d%d%d",&n,&m,&k)!=-1){memset(head,-1,sizeof(head));num=0;for(i=1;i<=n;i++)f[i]=i;for(i=0;i<m;i++){scanf("%d%d%d",&x,&y,&w);addedge(x,y,w);addedge(y,x,w);x=find(x);y=find(y);if(x!=y)f[x]=find(y);}memset(vis,0,sizeof(vis));ans=1;for(i=1;i<=n;i++){if(vis[i]==0){dis[i]=0;dfs[i]=ans;father[i]=i;dfs1(i);ans++;}}while(k--){scanf("%d%d",&x,&y);if(find(x)!=find(y)){printf("Not connected\n");continue;}j=LCA(x,y);printf("%d\n",j);}}return 0;}