【牛客小白月赛6】 C 桃花

题目地址：https://www.nowcoder.com/acm/contest/136/C

dfs找出最长路和次长路，将两个结果相加再加上起点即可；

#include<iostream>
using namespace std;

const int N = 1e6+5;
struct EDGE {
    int to, pre;
}edge[2*N];
int n, tot=1, head[N], first[N], second[N], ans=0;

void read(int &x) {
    int f = 1; x = 0;
    char ch = getchar();
   
    while (ch < '0' || ch > '9')   {if (ch == '-') f = -1; ch = getchar();}
    while (ch >= '0' && ch <= '9') {x = x * 10 + ch - '0'; ch = getchar();}
    x *= f;
}

void add_edge(int a, int b)
{
    edge[tot].to = b;
    edge[tot].pre = head[a];
    head[a] = tot++;
}

void dfs(int k, int pre)
{
    for(int i=head[k]; i; i=edge[i].pre) {
        int to = edge[i].to;
        if(to == pre) continue;
        
        dfs(to, k);
        if(first[k] < first[to]+1) {
            second[k] = first[k];
            first[k] = first[to]+1;
        }
        else
            second[k] = max(second[k], first[to]+1);
    }
    ans = max(ans, first[k]+second[k]);
}

int main()
{
    read(n);
    for(int a,b,i=1; i<n; i++) {
        read(a); read(b);
        add_edge(a,b);
        add_edge(b,a);
    }
    
    dfs(1, 0);
    cout<<ans+1<<endl;
    
    return 0;
}