CodeForces 1182D

图论的思维题，太秀了，网上答案也不多，我就也来bb吧

 总之47个样例姑且是过了，不知道还有没有反例；

会求树的重心和中心了，挺好

#include<cstdio>
#include<iostream>
#include<cstring>
#include<algorithm>
#include<vector>
#include<queue>
using namespace std;
#define maxn 100110
vector<int>G[maxn];
vector<int>an;
void insert(int be, int en) {
	G[be].push_back(en);
	G[en].push_back(be);
}

int root;
int n;
int in[maxn];
int de[maxn];
queue<int>que;
int find_root() {
	while (!que.empty()) {
		int x = que.front();
		que.pop();
		for (int i = 0; i < G[x].size(); i++){
			int p = G[x][i];
			in[p]--;
			if (in[p] == 1) {
				que.push(p);
				root = p;
			}
		}
	}
	return 0;
}

int cnt[maxn];
int flag = 0;
int jude(int x, int fa, int dep) {//这个点搜下去，是否符合条件
	if (cnt[dep] == 0) {
		cnt[dep] = de[x];
	}
	else {
		if (cnt[dep] != de[x]) {
			flag = 1;
			return 0;
		}
	}
	for (int i = 0; i < G[x].size(); i++) {
		int p = G[x][i];
		if (p == fa) continue;
		jude(p, x, dep + 1);
	}
	return 0;
}

int find_point(int x,int fa,int dep) {
	if (cnt[dep] == 0 && de[x] == 1) {
		cnt[dep] = 1;
		an.push_back(x);
	}
	for (int i = 0; i < G[x].size(); i++) {
		int p = G[x][i];
		if (p == fa) continue;
		if (de[p] == 2 || de[p] == 1) {
			find_point(p, x, dep + 1);
		}
		
	}
	return 0;
}
int main() {
	int be, en;
	scanf("%d", &n);
	for (int i = 1; i < n; i++) {
		scanf("%d %d", &be, &en);
		insert(be, en);
		de[be]++;
		de[en]++;
		in[be]++;
		in[en]++;
	}
	if (n == 1 || n == 2) {
		printf("1
");
		return 0;
	}
	for (int i = 1; i <= n; i++) {
		if (in[i] == 1) {
			in[i] = 0;
			que.push(i);
		}
	}
	/*if (n == 1 || n == 2) {
		printf("1
");
		return 0;
	}*/
	find_root();
	memset(cnt, 0, sizeof(cnt));
	find_point(root, root, 1);
	
	memset(cnt, 0, sizeof(cnt));
	flag = 0;
	jude(root, root, 1);
	
	if (!flag) {
		printf("%d
", root);
		return 0;
	}
	
	for (int i = 0; i < an.size(); i++) {
		memset(cnt, 0, sizeof(cnt));
		flag = 0;
		jude(an[i], an[i], 1);
		if (!flag) {
			printf("%d
", an[i]);
			return 0;
		}
	}
	printf("-1
");
	return 0;
}