有谁读懂了题意可以说题意啥意思呀？
尤其是下面这一句是什么意思？
Then for every (i,j) (i can be equal to j) one tree with the magic power ai-aj and one tree with the magic power ai+aj will appear in the forest, then you know there will be 3N trees with the magic power a1,…,a3na_1,…,a_{3n}a1,…,a3n.
为什么N棵树变成了3N棵?不是说任意一个(i,j)都会出现树$a_i-a_j$,$a_i+a_j$吗？
比如$1,10,100$三棵树，任意一个(i,j)对相减和相加，不是会出现$0,9,-9,99,-99,11,101,90,-90,110$的树吗，这样算上原来的都有13棵树，远比$3N=9$棵要多呀？
