树的直径

#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using ull = unsigned long long;
using pii = pair<int,int>;
using db = double;
int n;
const int N=1e5+10;
vector<int> a[N];
int mx=0,mx_node=0;
void dfs(int u,int fa,int d){
    if(d>mx){
        mx=d;
        mx_node=u;
    }
    for(auto v:a[u]){
        if(v==fa) continue;
        dfs(v,u,d+1);
    }
}
int main(){
    std::ios::sync_with_stdio(false),cin.tie(nullptr),cout.tie(nullptr);
    cin>>n;
    for(int i=1;i<n;i++){
        int u,v;
        cin>>u>>v;
        a[u].push_back(v);
        a[v].push_back(u);
    }
    dfs(1,-1,0);
    dfs(mx_node,-1,0);
    cout<<mx;
    return 0;
}