CodeForces - 764C

C. Timofey and a tree
time limit per test
2 seconds
memory limit per test
256 megabytes
input
standard input
output
standard output

Each New Year Timofey and his friends cut down a tree of n vertices and bring it home. After that they paint all the n its vertices, so that the i-th vertex gets color ci.

Now it's time for Timofey birthday, and his mother asked him to remove the tree. Timofey removes the tree in the following way: he takes some vertex in hands, while all the other vertices move down so that the tree becomes rooted at the chosen vertex. After that Timofey brings the tree to a trash can.

Timofey doesn't like it when many colors are mixing together. A subtree annoys him if there are vertices of different color in it. Timofey wants to find a vertex which he should take in hands so that there are no subtrees that annoy him. He doesn't consider the whole tree as a subtree since he can't see the color of the root vertex.

A subtree of some vertex is a subgraph containing that vertex and all its descendants.

Your task is to determine if there is a vertex, taking which in hands Timofey wouldn't be annoyed.

Input

The first line contains single integer n (2 ≤ n ≤ 105) — the number of vertices in the tree.

Each of the next n - 1 lines contains two integers u and v (1 ≤ u, v ≤ n, u ≠ v), denoting there is an edge between vertices u and v. It is guaranteed that the given graph is a tree.

The next line contains n integers c1, c2, ..., cn (1 ≤ ci ≤ 105), denoting the colors of the vertices.

Output

Print "NO" in a single line, if Timofey can't take the tree in such a way that it doesn't annoy him.

Otherwise print "YES" in the first line. In the second line print the index of the vertex which Timofey should take in hands. If there are multiple answers, print any of them.

Examples
Input
4
1 2
2 3
3 4
1 2 1 1
Output
YES
2
Input
3
1 2
2 3
1 2 3
Output
YES
2
Input
4
1 2
2 3
3 4
1 2 1 2
Output
NO
题目大意就是说是否存在这么一个点，使得去掉这个点后，所形成的子树在同一棵树上是否颜色相同，如果有不同的就输出NO，有就输出yes，并且输出该节点，答案可能不唯一。
思路：DFS ，邻接表，优化。
ps：该方法不是最优的方法，在这里我只写了这个大多数人都能想到的方法。

AC代码：
 1 #include<iostream>
 2 #include<stdio.h>
 3 #include<vector>
 4 using namespace std;
 5 struct Node{
 6     int st;
 7     int en;
 8 } dis[100050];
 9 vector<int > load[100050];//邻接表
10 int color[100050];
11 bool dfs(int root,int fa,int col,int t)
12 {
13     if(t!=2) t++;
14     else
15     {
16         if(col!=color[fa])//颜色不同，返回false
17             return false;
18     }
19     bool fl;
20     for(int i=0;i<load[root].size();i++)
21     {
22         int nxt=load[root][i];
23         if(nxt!=fa)
24         {
25             fl=dfs(nxt,root,color[nxt],t);
26             if(!fl)
27                 return false;
28         }
29     }
30     return true;
31 }
32 int main()
33 {
34     int n,x,y;
35     cin>>n;
36     for(int i=0;i<n-1;i++)
37     {
38         scanf("%d%d",&x,&y);
39         dis[i].st=x;//这个数组是用来查找的时候优化的。
40         dis[i].en=y;
41         load[x].push_back(y);
42         load[y].push_back(x);
43     }
44     for(int i=1;i<=n;i++)
45         scanf("%d",&color[i]);
46     bool flag=false,fg=false;
47     int i,vis=0;
48     for(int i=0;i<n-1;i++)
49         if(color[dis[i].st]==color[dis[i].en])
50             vis++;
51     if(vis==n-1)//如果相等，说明都是同一种颜色。输出1就ok
52     {
53         cout<<"YES"<<endl<<1<<endl;
54         return 0;
55     }
56     if(n==2)//只有两个节点的时候
57     {
58         cout<<"YES"<<endl<<1<<endl;
59         return 0;
60     }
61     int r;
62     for(int j=0;j<n-1;j++)
63     {
64         int i=dis[j].st;
65         int b=dis[j].en;
66         if(color[i]!=color[b])//颜色不同，必有一个点是要去除的点。
67         {
68             flag=dfs(i,i,color[i],0);
69             if(flag)
70             {
71                 r=i;
72                 fg=true;
73                 break;
74             }
75             flag=dfs(b,b,color[b],0);
76             if(flag)
77             {
78                 fg=true;
79                 r=b;
80             }
81             break;
82         }
83     }
84     if(fg)
85         cout<<"YES"<<endl<<r<<endl;
86     else    //如果颜色不同，且fg为false，那么必不存在
87         cout<<"NO"<<endl;
88     return 0;
89 }
90

 




posted @ 2017-07-25 21:48  ISGuXing  阅读(353)  评论(0)  编辑  收藏  举报