Balanced Removals (Easier)

题意翻译

给定 $n$ 个三维坐标点 $(x_i,y_i,z_i)$。 每次你可以删除两个点 $i,j$ 当且仅当不存在一个 $k$ 同时满足： $\min(x_i,x_j)\le x_k\le \max(x_i, x_j),\min(y_i,y_j)\le y_k\le \max(y_i, y_j),\min(z_i,z_j)\le z_k\le \max(z_i, z_j)$。 保证点的坐标互不相同，你需要给出一个删除所有点的方案。 注意删除点之后的点不会成为合法的$k$

题目描述

This is an easier version of the problem. In this version, $ n \le 2000 $ . There are $ n $ distinct points in three-dimensional space numbered from $ 1 $ to $ n $ . The $ i $ -th point has coordinates $ (x_i, y_i, z_i) $ . The number of points $ n $ is even. You'd like to remove all $ n $ points using a sequence of $ \frac{n}{2} $ snaps. In one snap, you can remove any two points $ a $ and $ b $ that have not been removed yet and form a perfectly balanced pair. A pair of points $ a $ and $ b $ is perfectly balanced if no other point $ c $ (that has not been removed yet) lies within the axis-aligned minimum bounding box of points $ a $ and $ b $ . Formally, point $ c $ lies within the axis-aligned minimum bounding box of points $ a $ and $ b $ if and only if $ \min(x_a, x_b) \le x_c \le \max(x_a, x_b) $ , $ \min(y_a, y_b) \le y_c \le \max(y_a, y_b) $ , and $ \min(z_a, z_b) \le z_c \le \max(z_a, z_b) $ . Note that the bounding box might be degenerate. Find a way to remove all points in $ \frac{n}{2} $ snaps.

输入输出格式

输入格式

The first line contains a single integer $ n $ ( $ 2 \le n \le 2000 $ ; $ n $ is even), denoting the number of points. Each of the next $ n $ lines contains three integers $ x_i $ , $ y_i $ , $ z_i $ ( $ -10^8 \le x_i, y_i, z_i \le 10^8 $ ), denoting the coordinates of the $ i $ -th point. No two points coincide.

输出格式

Output $ \frac{n}{2} $ pairs of integers $ a_i, b_i $ ( $ 1 \le a_i, b_i \le n $ ), denoting the indices of points removed on snap $ i $ . Every integer between $ 1 $ and $ n $ , inclusive, must appear in your output exactly once. We can show that it is always possible to remove all points. If there are many solutions, output any of them.

输入输出样例

输入样例 #1

6
3 1 0
0 3 0
2 2 0
1 0 0
1 3 0
0 1 0

输出样例 #1

3 6
5 1
2 4

输入样例 #2

8
0 1 1
1 0 1
1 1 0
1 1 1
2 2 2
3 2 2
2 3 2
2 2 3

输出样例 #2

4 5
1 6
2 7
3 8

说明

In the first example, here is what points and their corresponding bounding boxes look like (drawn in two dimensions for simplicity, as all points lie on $ z = 0 $ plane). Note that order of removing matters: for example, points $ 5 $ and $ 1 $ don't form a perfectly balanced pair initially, but they do after point $ 3 $ is removed.