It’s universally acknowledged that there’re innumerable trees in the campus of HUST. One day you find an endless forest consisting of magic trees. The forest can be described in such a strange way: initially you know N trees with the magic power a1,…,an. Then for every (i,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,…,a3n. This process can be done repeatedly,so you know that the infinite forest contains innumerable trees. What you want to know is if there’s a tree with the magic power X.
It’s universally acknowledged that there’re innumerable trees in the campus of HUST. One day you find an endless forest consisting of magic trees. The forest can be described in such a strange way: initially you know N trees with the magic power a1,…,an. 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. This process can be done repeatedly, so you know that the infinite forest contains innumerable trees. What you want to know is if there’s a tree with the magic power X.
（图片来源网络，侵删）
