CodeForces 1182D

Posted 2022-11-02 lesning

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\\n");
		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\\n");
		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\\n", 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\\n", an[i]);
			return 0;
		
	
	printf("-1\\n");
	return 0;