hihocoder #1224 : 赛车 dfs

题目链接:

http://hihocoder.com/problemset/problem/1224

题意:

题解:

首先我们先dfs出最长链，然后我们再dfs出每一个点能够最长延展多少
然后最后答案就是最长链的长度+最长延展多少就好了

代码:

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 typedef long long ll;
 4 #define MS(a) memset(a,0,sizeof(a))
 5 #define MP make_pair
 6 #define PB push_back
 7 const int INF = 0x3f3f3f3f;
 8 const ll INFLL = 0x3f3f3f3f3f3f3f3fLL;
 9 inline ll read(){
10     ll x=0,f=1;char ch=getchar();
11     while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
12     while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
13     return x*f;
14 }
15 //////////////////////////////////////////////////////////////////////////
16 const int maxn = 1e5+10;
17 
18 vector<int> G[maxn];
19 int d[maxn],vis[maxn],fa[maxn];
20 
21 void dfs(int x,int deep){
22     d[x] = deep;
23     for(auto v : G[x]){
24         if(vis[v]) continue;
25 
26         fa[v] = x;
27         dfs(v,deep+1);
28     }
29 }
30 
31 int main(){
32     int n=read();
33     for(int i=1; i<n; i++){
34         int u=read(),v=read();
35         G[u].push_back(v);
36     }
37     dfs(1,0);
38 
39     int bf=1,ans1 = 0;
40     for(int i=1; i<=n; i++){
41         if(d[i] > ans1){
42             ans1 = d[i];
43             bf = i;
44         }
45     }
46 
47     while(bf != 1){
48         vis[bf] = 1;
49         bf = fa[bf];
50     }
51     vis[1] = 1;
52 
53     for(int i=1; i<=n; i++){
54         if(vis[i])
55             dfs(i,0);
56     }
57 
58     int ans2 = 0;
59     for(int i=1; i<=n; i++)
60         if(!vis[i])
61             ans2 = max(d[i],ans2);
62 
63     cout << ans1+ans2 << endl;
64 
65     return 0;
66 }

 

posted @ 2017-02-28 02:16  _yxg123  阅读(97)  评论(0)  编辑  收藏  举报