超

#include <bits/stdc++.h> 
#define int long long
using namespace std;

const int maxn = 300005;

int fa[maxn], len[maxn];

int find(int x) {
	return x == fa[x]? x : fa[x] = find(fa[x]);
}

void un(int x, int y) {
	int fx = find(x), fy = find(y);
	if (fx != fy) {
		fa[fx] = fy;
		len[fy] = max(max(len[fx], len[fy]),(len[fx]+1)/2 + (len[fy]+1)/2 + 1); 
	}
}


vector <int> g[maxn]
int l, u;

void findfur(int x, int fa, int s) {
	if (s >= l) l = s, u = x;
	for (int i = 0; i < g[x].size(); i++) {
		int y = g[x][i];
		if (y ^ fa) findfur(y, x, s+1);
	}
}

int gd(int st) {
	l = 0; findfur(st, 0, 0);
	l = 0; findfur(u, 0, 0);
	return l;
}

signed main() {
	cin.tie(0); ios::sync_with_stdio(0);
	int n, m, q;
	cin >> n >> m >> q;
	for (int i = 1; i <= n; i++) {
		fa[i] = i;
		// len[i] = 1;
	}
	for (int i = 1; i <= m; i++) {
		int x, y;
		cin >> x >> y;
		g[x].push_back(y); g[y].push_back(x);
		fa[find(x)] = find(y);
	}
	for (int i = 1; i <= n; i++) if (i == fa[i]) len[i] = gd(i);
	while (q--) {
		int opt, x, y;
		cin >> opt;
		if (opt == 1) {
			cin >> x;
			cout << len[find(x)] << "\n";
		} else {
			cin >> x >> y;
			un(x, y);
			//cout << len[find(x)] << "\n"; 
		}
	}
}

显然，错误在于vector那一行（22行）没加分号。

然鹅

emm……一开始我觉得可能是Dev的BUG。

然鹅你谷IDE的编译出来也是这样。

有无大佬解释。