【牛客小白月赛6】 C 桃花 - 树上最长路

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+;
 struct EDGE {
     int to, pre;
 }edge[*N];
 int n, tot=, head[N], first[N], second[N], ans=;

 void read(int &x) {
     int f = ; x = ;
     char ch = getchar();

     while (ch < '' || ch > '')   {if (ch == '-') f = -; ch = getchar();}
     while (ch >= '' && ch <= '') {x = x *  + ch - ''; 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]+) {
             second[k] = first[k];
             first[k] = first[to]+;
         }
         else
             second[k] = max(second[k], first[to]+);
     }
     ans = max(ans, first[k]+second[k]);
 }

 int main()
 {
     read(n);
     for(int a,b,i=; i<n; i++) {
         read(a); read(b);
         add_edge(a,b);
         add_edge(b,a);
     }

     dfs(, );
     cout<<ans+<<endl;

     return ;
 }