How to get the smallest in lexicographical order?

The smallest lexicographical order is an order relation where string s is smaller than t, given the first character of s (s₁) is smaller than the first character of t (t₁), or in case they are equivalent, the second character, etc.

So aaabbb is smaller than aaac because although the first three characters are equal, the fourth character b is smaller than the fourth character c.

For cbacdcbc, there are several options, since b and c are duplicates, you can decided which duplicates to remove. This results in:

cbacdcbc = adbc
cbacdcbc = adcb
cbacdcbc = badc
cbacdcbc = badc
...

since adbc < adcb, you cannot thus simply answer with the first answer that pops into your mind.