PAT a1021题解

#include <cstdio>
#include <cstring>
#include <vector>
#include <algorithm>

using namespace std;

const int N = 100010;
vector<int> G[N];
bool isRoot[N];
int father[N];

int findFather(int x){
	int a = x;
	while(x != father[x]){
		x = father[x];
	}
	while(a != father[a]){
		int z = a;
		a = father[a];
		father[z] = x;
	}
	return x;
}

void Union(int a, int b){
	int faA = findFather(a);
	int faB = findFather(b);
	if(faA != faB){
		father[faA] = faB;
	}
}

void init(int n){
	for(int i = 1; i <= n; i++){
		father[i] = i;
	}
} 

int calBlock(int n){
	int Block = 0;
	for(int i = 1; i <= n; i++){
		isRoot[findFather(i)] = true;
	}
	for(int i = 1; i <= n; i++){
		Block += isRoot[i];
	}
	return Block;
}

int maxH = 0;
vector<int> temp, Ans;

void DFS(int u, int Height, int pre){
	if(Height > maxH){
		temp.clear();
		temp.push_back(u);
		maxH = Height;
	} else if(Height == maxH){
		temp.push_back(u);
	}
	for(int i = 0; i < G[u].size(); i++){
		if(G[u][i] == pre) continue;
		DFS(G[u][i], Height + 1, u);
	}
} 

int main(){
	int a, b, n;
	scanf("%d", &n);
	init(n);
	for(int i = 1; i < n; i++){
		scanf("%d%d", &a, &b);
		G[a].push_back(b);
		G[b].push_back(a);
		Union(a, b);
	}
	int Block = calBlock(n);
	if(Block != 1){
		printf("Error: %d components\n", Block);
	} else{
		DFS(1, 1, -1);
		Ans = temp;
		DFS(Ans[0], 1, -1);
		for(int i = 0; i < temp.size(); i++){
			Ans.push_back(temp[i]);
		}
		sort(Ans.begin(), Ans.end());
		printf("%d\n", Ans[0]);
		for(int i = 1; i < Ans.size(); i++){
			if(Ans[i] != Ans[i - 1]){
				printf("%d\n", Ans[i]);
			}
		}
	}
	return 0;
}