hihoCoder week11 树中的最长路

题目链接： https://hihocoder.com/contest/hiho11/problem/1

求树中节点对 距离最远的长度

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

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

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

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

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

int dfs(int u,int fa)
{
    int l1=-,l2=-;
    for(int i=; 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 + );
    // cout << u <<" 最长的路为" <<l1 <<endl;
    return l1+;
}

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