cf1182D Complete Mirror

• 可以得到一个结论， 可行的点要么是直径端点， 要么是直径中点， 要么是直径中点引出的链中最短的端点

#include<cstdio>
#include<algorithm>
#include<iostream>
#include<cstring>
#include<queue>
#define mmp make_pair
#define ll long long
#define M 100010
using namespace std;
int read() {
	int nm = 0, f = 1;
	char c = getchar();
	for(; !isdigit(c); c = getchar()) if(c == '-') f = -1;
	for(; isdigit(c); c = getchar()) nm = nm * 10 + c - '0';
	return nm * f;
}
vector<int> to[M];
int n;
int sta[M], tp, a, b, mid, maxx;
int du[M], deep[M], af[M];
vector<int> note[M];

void dfss(int now, int f) {
	deep[now] = deep[f] + 1;
	note[deep[now]].push_back(du[now]);
	for(int i = 0; i < to[now].size(); i++) {
		int vj = to[now][i];
		if(vj == f) continue;
		dfss(vj, now);
	}
}

void dfs(int now, int f) {
	sta[++tp] = now;
	if(tp > maxx) {
		maxx = tp;
		b = now, mid = sta[(tp + 1) >> 1];
	}
	for(int i = 0; i < to[now].size(); i++) {
		int vj = to[now][i];
		if(vj == f) continue;
		dfs(vj, now);
	}
	tp--;
}

void check(int x) {
	for(int i = 1; i <= n; i++) vector<int>().swap(note[i]);
	dfss(x, 0);
	for(int i = 1; i <= n; i++) {
		for(int j = 1; j < note[i].size(); j++) {
			if(note[i][j] != note[i][j - 1]) return;
		}
	}
	cout << x << "\n";
	exit(0);
}

void work(int now, int f) {
	deep[now] = deep[f] + 1;
	if(du[now] > 2) return;
	if(du[now] == 1) {
		if(maxx > deep[now]) {
			maxx = deep[now], a = now;
		}
		return;
	}
	for(int i = 0; i < to[now].size(); i++) {
		int vj = to[now][i];
		if(vj == f) continue;
		work(vj, now);
	}
}

int main() {
	n = read();
	for(int i = 1; i < n; i++) {
		int vi =  read(), vj = read();
		to[vi].push_back(vj);
		to[vj].push_back(vi);
		du[vi]++;
		du[vj]++;
	}
	a = 1;
	dfs(1, 0);
	maxx = 0;
	a = b;
	dfs(a, 0);
	check(a);
	check(b);
	check(mid);
	maxx = 0x3e3e3e3e;
	for(int i = 0; i < to[mid].size(); i++) {
		int vj = to[mid][i];
		work(vj, mid);
	}
	check(a);
	cout << "-1\n";
	return 0;
}