牛客小白月赛39 F 孤独

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
int ans = INT_MAX, n;
int sz[1000006];
int dp[1000006];
vector<int>vec[1000006];
void dfs(int i, int fa) {
	sz[i] = 1;
	int mx1 = 0;
	int mx2 = 0;
	int mx3 = 0; //第三大的子树
	for (auto j : vec[i]) {
		if (j == fa)continue;
		dfs(j, i);
		sz[i] += sz[j];
		if (sz[j] > sz[mx1]) {
			mx3 = mx2;
			mx2 = mx1;
			mx1 = j;
		} else if (sz[j] > sz[mx2]) {
			mx3 = mx2;
			mx2 = j;
		} else if (sz[j] > sz[mx3]) {
			mx3 = j;
		}
		dp[i] = max(dp[mx1], sz[mx2]);
	}
	ans = min(ans, max(n - sz[i], max(dp[mx1], max(dp[mx2], sz[mx3]))));
}
int main() {
	int i, j, x, y;
	scanf("%d", &n);
	for (i = 1; i < n; i++) {
		scanf("%d%d", &x, &y);
		vec[x].push_back(y);
		vec[y].push_back(x);
	}
	dfs(1, 0);
	printf("%d\n", ans);
	return 0;
}