造一棵树

#include <cstdio>
#include <vector>
#include <utility>
#include <cassert>
#include <cstdlib>
#include <fstream>
#include <iostream>
#include <algorithm>

#define hb bh

using namespace std;

const int maxn = 1000000;

int n, m, seed;
int bh[maxn];
vector<pair<int, int> > ve;
vector<int> rt;
vector<int> ln;

int main(int argc, char** argv) {
	assert(argc == 4);
	ofstream fout(argv[1]);
	sscanf(argv[2], "%d", &n);
	sscanf(argv[3], "%d", &seed);
	srand((unsigned) seed);
	fout << n << '\n';
	mt19937 rnd((unsigned) seed);
	int lim = n / 3;
	for (int i = 1; i <= n; ++i) {
		bh[i] = i;
	}
	int Lrt = max(1, max(lim - (lim / 10), lim - 10));
	random_shuffle(bh + 1, bh + n + 1);
	rt.push_back(bh[1]);
	for (int i = 2; i <= lim; ++i) {
		ve.push_back(make_pair(bh[i], rt[rnd() % rt.size()]));
		if (i - Lrt > 1) {
			rt.push_back(hb[i - Lrt]);
		}
	}
	for (int i = lim + 1; i <= n; ++i) {
		if (ln.empty() || !(rnd() % max(3, lim >> 3))) {
			ve.push_back(make_pair(hb[i], rt[rnd() % rt.size()]));
		} else {
			ve.push_back(make_pair(hb[i], ln[max(0, (int) (ln.size() - (rnd() % 100) - 1))]));
		}
		ln.push_back(hb[i]);
	}
	random_shuffle(ve.begin(), ve.end());
	for (auto x : ve) {
		if (rnd() & 1) {
			fout << x.first << ' ' << x.second << '\n';
		} else {
			fout << x.second << ' ' << x.first << '\n';
		}
	}
	fout.close();
	return 0;
}