树形dp【树的直径】

bfs

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#include<bits/stdc++.h>
using namespace std;
const int N=1e5+10;
#define LL long long
int cnt,h[N],to[N*2],w[N*2],ne[N*2];
void add(int a,int b,int c){
  ne[++cnt]=h[a];
  w[cnt]=c;
  to[cnt]=b;
  h[a]=cnt;
}
LL d[N],mx,vis[N];
void bfs(int x){
  memset(d,0,sizeof d);
  memset(vis,0,sizeof vis);
  queue<LL> q;
  q.push(x);
  while(q.size()){
   LL t=q.front();
   q.pop();
   if(vis[t]) continue;
   vis[t]=1;
   for(int i=h[t];i;i=ne[i]){
     int w1=w[i],v=to[i];
     if(d[v]<d[t]+w1&&!vis[v]){
       d[v]=d[t]+w1;
       if(d[v]>d[mx]) mx=v;
       q.push(v);
     }
   } 
  }
}

int main(){
  int n;
  LL ans=0;
  cin>>n;
  for(int i=1;i<n;i++){
    int a,b,c;
    cin>>a>>b>>c;
    ans+=1LL*c*2;
    add(a,b,c);
    add(b,a,c);
  }
  bfs(1);
  bfs(mx);
  ans-=d[mx];
  cout<<ans;
  return 0;
}

dfs

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const int N = 10000 + 10;

int n, d = 0;
int d1[N], d2[N];
vector<int> E[N];

void dfs(int u, int fa) {
  d1[u] = d2[u] = 0;
  for (int v : E[u]) {
    if (v == fa) continue;
    dfs(v, u);
    int t = d1[v] + 1;
    if (t > d1[u])
      d2[u] = d1[u], d1[u] = t;
    else if (t > d2[u])
      d2[u] = t;
  }
  d = max(d, d1[u] + d2[u]);
}

int main() {
  scanf("%d", &n);
  for (int i = 1; i < n; i++) {
    int u, v;
    scanf("%d %d", &u, &v);
    E[u].push_back(v), E[v].push_back(u);
  }
  dfs(1, 0);
  printf("%d\n", d);
  return 0;
}