B09 DFS 树的中心

视频链接：E34 树形DP 树的中心_哔哩哔哩_bilibili

#include <iostream>
#include <cstring>
#include <algorithm>
#include <vector>
using namespace std;

const int N=20010;
int n,a,b,c,ans=2e9;
struct edge{int v,w;};
vector<edge> e[N];
int d1[N],d2[N],path[N],up[N];

void dfs_d(int x,int fa){
  for(auto ed : e[x]){
    int y=ed.v, z=ed.w;
    if(y==fa) continue;
    dfs_d(y, x);
    if(d1[y]+z>d1[x]) 
      d2[x]=d1[x],d1[x]=d1[y]+z,path[x]=y;
    else if(d1[y]+z>d2[x]) d2[x]=d1[y]+z;
    // printf("回%d d1=%d d2=%d\n",x,d1[x],d2[x]);
  }
}
void dfs_up(int x,int fa){
  for(auto ed : e[x]){
    int y=ed.v, z=ed.w;
    if(y==fa) continue;
    if(y==path[x])up[y]=max(up[x],d2[x])+z;
    else up[y]=max(up[x],d1[x])+z;
    // printf("下%d up=%d\n",y,up[y]);
    dfs_up(y, x);
  }
}
int main(){
  cin>>n;
  for(int i=1; i<n; i++){
    cin>>a>>b>>c;
    e[a].push_back({b,c});
    e[b].push_back({a,c});
  }
  dfs_d(1, 0);
  dfs_up(1, 0);
  for(int i=1; i<=n; i++)
    ans=min(ans,max(d1[i],up[i]));
  cout << ans << endl;
  return 0;
}