树中的最长路(hihocoder 第十一周)

 1 #include "iostream"
 2 #include "vector"
 3 #include "memory.h"
 4 
 5 using namespace std;
 6 //建立图
 7 vector<int> m[111111];
 8 int ans;
 9 //枚举所有的根节点-from
10 int dfs(int from,int node)
11 {
12     int first = 0,second = 0;
13     int temp = 0;
14     for (int i = 0;i < m[node].size();++ i)
15     {
16         if (m[node][i] != from)
17         {
18             temp = dfs(node,m[node][i]);
19             if (temp > first)  {
20                 second = first;
21                 first = temp;
22             }
23             else if (temp > second) second = temp;
24         }
25     }
26     if (first + second > ans)
27         ans = first + second;
28     return first + 1;
29 }
30 int main()
31 {
32     int n, x, y;
33     cin >> n;
34     n--;
35     while (n--)
36     {
37         cin >> x >> y;
38         m[x].push_back(y);
39         m[y].push_back(x);
40     }
41     dfs(0,1);
42     cout << ans << endl;
43     return 0;
44 }