So this is my code to ABC279F:

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

int fa[600005];

void init() {
	memset(fa, -1, sizeof fa);
}

int find_root(int x) {
	if (fa[x] == -1) {
		return x;
	}
	return fa[x] = find_root(fa[x]);
}

void unite(int x, int y) {
	if (find_root(x) != find_root(y) && find_root(x) != -1) {
		fa[find_root(x)] = find_root(y);
	}
}

int now[300005], idx[300005], past[600005];

int main() {
	init();
	int n, q;
	scanf("%d %d", &n, &q);
	for (int i = 0; i < n; i++) {
		now[i] = i;
		past[i] = i;
		idx[i] = i;
	}
	int cntn = n - 1;
	int cnt = n - 1;
	while (q--) {
		int tp;
		scanf("%d", &tp);
		if (tp == 1) {
			int x, y;
			scanf("%d %d", &x, &y);
			x--, y--;
			unite(now[y], now[x]);
			now[y] = ++cntn;
			past[cntn] = y;
		} else if (tp == 2) {
			int x;
			scanf("%d", &x);
			x--;
			idx[++cnt] = now[x];
		} else {
			int x;
			scanf("%d", &x);
			x--;
			printf("%d\n", past[find_root(idx[x])] + 1);
		}
//		for (int i = 0; i < n; i++) {
//			cerr << now[i] << " ";
//		}
//		cerr << endl;
	}
	return 0;
}

It should be RE (Though the size of "idx" is 300005), but it judged as WA.

Why?

And it wasted me a lot of time :(