題目連結： https://hihocoder.com/contest/hiho11/problem/1

求樹中節點對 距離最遠的長度

#include <bits/stdc++.h>
using namespace std;

const int N = 1e5 + 10;
int n;
vector<int> G[N];
int d[N];
void dfs(int u, int fa)
{
    for(int i=0; i<G[u].size(); i++) {
        int v = G[u][i];
        if
 
(v != fa) {
            d[v] = d[u] + 1;
            dfs(v, u);
        }
    }
}

int main()
{
    freopen("in.txt", "r", stdin);
    scanf("%d",&n);
    for(int i=0; i<n-1; i++) {
        int a,b; scanf("%d %d", &a, &b);
        G[a].push_back(b);
        G[b].push_back(a);
    }
    d[
 
1] = 0;
    dfs(1, 0);
    int mx =  0;
    int index = -1;
    for(int i=1; i<=n; i++) {
        if(mx < d[i]) {
            mx = d[i];
            index = i;
        }
    }
    //printf("%d\n", mx);   
    memset(d,0,sizeof(d));
    dfs(index, 0);
    for(int i=1; i<=n; i++) {
        
 
if(mx < d[i]) {
            mx = d[i];
            index = i;
        }
    }
    cout << mx <<endl;                                                                                     
    return 0;
}

#include <bits/stdc++.h>
using namespace std;

const int N = 1e5 + 10;
vector<int> G[N];
int n, ans;

int dfs(int u,int fa)
{
    int l1=-1,l2=-1;
    for(int i=0; i<G[u].size(); i++) {
        int v = G[u][i]; 
        if(v == fa) continue;
        int l = dfs(v, u);
        if(l > l1) 
            l2=l1 , l1=l;
        else 
            l2 = max(l2, l);
        
    }
    //cout << u <<" "<< l1 <<" "<< l2 <<endl;
    ans = max(ans , l1 + l2 + 2);
    // cout << u <<" 最長的路為" <<l1 <<endl;
    return l1+1;
}


int main()
{
    //freopen("in.txt", "r", stdin);
    cin >> n;
    for(int i=1; i<n; i++) {
        int a,b; scanf("%d %d", &a, &b);
        G[a].push_back(b);
        G[b].push_back(a);
    }
    ans = 0;
    dfs(1,0);
    cout << ans <<endl;
    return 0;
}