Anyone who could help me find the fastest way to calculate the n-th term of the sequence below? please

Question

1 2 1 2 3 2 1 2 3 4 3 2 1 2 3 4 5 4 3 2 1 2 3 4 5 6 5 4 3 2 1 2 3 4 5 6 7 6 5 4 3 2 1.....

Answer 1

sounds like homework. we don't do homework, but if you present code your working on we will help with that