CHEFTWOS why is this a good string?

IN this question

how is this a valid/good strings

1 1 1
2 2 1

as in the example test case it is explained that {1,2,3} is a good pair of strings

Chef's dog wants to make both the strings a and b good by choosing some subset S (possibly empty) of indices of set {1, 2, ..., n} and swapping each index i ϵ S between string a and b, i.e. swapping ai and bi.

Subset is subset of indices to be swapped. The string length remains the same.

2211 and 1111 are good strings as its possible to start from start, travel every char once, and reach end.