codeforces 690C3 Brain Network

simple:并查集一下

#include <vector>
#include <iostream>
#include <queue>
#include <cmath>
#include <map>
#include <cstring>
#include <algorithm>
#include <cstdio>
using namespace std;
typedef long long LL;
const int N=2e5+;
const LL mod=1e9+;
int fa[N];
int find(int x){
  return x==fa[x]?x:fa[x]=find(fa[x]);
}
int main(){
    int n,m;
    scanf("%d%d",&n,&m);
    for(int i=;i<=n;++i)fa[i]=i;
    while(m--){
      int u,v;
      scanf("%d%d",&u,&v);
      u=find(u),v=find(v);
      if(u!=v)fa[u]=v,--n;
      else{printf("no\n");return ;}
    }
    if(n==)printf("yes\n");
    else printf("no\n");
    return ;
}

medium:最长路

#include <vector>
#include <iostream>
#include <queue>
#include <cmath>
#include <map>
#include <cstring>
#include <algorithm>
#include <cstdio>
using namespace std;
typedef long long LL;
const int N=2e5+;
const LL mod=1e9+;
int head[N],tot,n,m,d[N>>],ret;
struct Edge{
  int v,next;
}edge[N];
void add(int u,int v){
  edge[tot].v=v;
  edge[tot].next=head[u];
  head[u]=tot++;
}
void bfs(int u){
   memset(d,-,sizeof(d));
   queue<int>q;q.push(u);
   d[u]=;
   while(!q.empty()){
     int x=q.front();
     q.pop();
     for(int i=head[x];~i;i=edge[i].next){
         int v=edge[i].v;
         if(d[v]!=-)continue;
         d[v]=d[x]+;
         q.push(v);
         if(d[v]>d[ret])ret=v;
     }
   }
}
int main(){
   scanf("%d%d",&n,&m);
   memset(head,-,sizeof(head));tot=;
   for(int i=;i<n;++i){
     int u,v;
     scanf("%d%d",&u,&v);
     add(u,v);add(v,u);
   }
   ret=;
   bfs();bfs(ret);
   printf("%d\n",d[ret]);
   return ;
}

hard:动态最长路,LCA维护

#include <vector>
#include <iostream>
#include <queue>
#include <cmath>
#include <map>
#include <cstring>
#include <algorithm>
#include <cstdio>
using namespace std;
typedef long long LL;
const int N=2e5+;
const LL mod=1e9+;
int head[N],tot,n,d[N];
struct Edge{
  int v,next;
}edge[N];
void add(int u,int v){
  edge[tot].v=v;
  edge[tot].next=head[u];
  head[u]=tot++;
}
int fa[N][];
void dfs(int u,int f){
   fa[u][]=f;d[u]=d[f]+;
   for(int i=head[u];~i;i=edge[i].next)dfs(edge[i].v,u);
}
int LCA(int u,int v){
   if(d[u]<d[v])swap(u,v);
   for(int t=d[u]-d[v],i=;t;t>>=,++i)
    if(t&)u=fa[u][i];
   if(u==v)return u;
   for(int i=;i>=;--i){
     if(fa[u][i]!=-&&fa[u][i]!=fa[v][i])
      u=fa[u][i],v=fa[v][i];
   }
   return fa[u][];
}
int main(){
   scanf("%d",&n);
   memset(head,-,sizeof(head));
   for(int i=;i<=n;++i){
     int u;scanf("%d",&u);add(u,i);
   }
   memset(fa,-,sizeof(fa));
   dfs(,);fa[][]=-;
   for(int j=;(<<j)<=n;++j)
    for(int i=;i<=n;++i)
      if(fa[i][j-]!=-)
        fa[i][j]=fa[fa[i][j-]][j-];
   printf("");
   int x=,y=,z=;
   for(int i=;i<=n;++i){
      int tpx=LCA(x,i),lenx=d[x]+d[i]-*d[tpx];
      int tpy=LCA(y,i),leny=d[y]+d[i]-*d[tpy];
      if(lenx>=leny&&lenx>=z){
         y=i;z=lenx;
      }
      else if(leny>=lenx&&leny>=z){
         x=i;z=leny;
      }
      printf(" %d",z);
   }
   printf("\n");
   return ;
}