https://bbs.emath.ac.cn/thread-17154-1-1.html
Place two 1's, two 2's, ..., two n's into 2n+1 cells(one would be left empty), s.t. between each pair of same number k is k cell, e.g. n=1, solution is [1,empty,1].
I solved n=20 but took like a month
Place two 1's, two 2's, ..., two n's into 2n+1 cells(one would be left empty), s.t. between each pair of same number k is k cell, e.g. n=1, solution is [1,empty,1].
I solved n=20 but took like a month