Sorted Groups

You are given a string $S$ of length $N$ consisting of lowercase English letters. Positions are numbered from $1$ to $N$. You must partition the set of positions $\{1, 2, \ldots, N\}$ into two groups $A$ and $B$ (either group may be empty). Then, for each group independently, perform the following operation: - Collect the characters present at the group's positions. - Sort these characters in non