Sorting — Sort Characters by Frequency

solve(s) — return the characters of s sorted by decreasing frequency. Every
occurrence of a character stays grouped: "eert", not "erte".

Ties are broken by the character itself, ascending by character code, so there is
exactly one answer (uppercase letters sort before lowercase ones).

Count with a map, then sort the distinct characters — that is O(n + k log k) with
k distinct characters — and rebuild the string.

Examples

solve("tree")    → "eert"
solve("cccaaa")  → "aaaccc"     // tie: "a" before "c"
solve("Aabb")    → "bbAa"       // "A" (65) before "a" (97)