PPractick

The problem

A text-analytics widget displays a word with its most frequent characters first: "tree" becomes "eert", because 'e' appears twice and the other letters once. Every copy of a character stays together. The natural approach counts each character, then orders characters by count, most frequent first. For a deterministic display, characters with equal counts appear in increasing character order. A bucket sort by frequency brings it down to linear time.

Example: "tree" → "eert"; "cccaaa" → "aaaccc".

Function signature

string frequencySort(const string& s);  // higher frequency first; ties by character code, ascending

Test cases

Input s	Output	Explanation
"tree"	"eert"	e×2, then r and t (r < t)
"cccaaa"	"aaaccc"	Tie at 3: 'a' comes before 'c'
"Aabb"	"bbAa"	b×2, then 'A' (65) before 'a' (97)
"z"	"z"
""	""

(Confirm: characters are ASCII; case-sensitive; LeetCode accepts any order among ties, while this version fixes it for determinism.)

How interviewers phrase it

• "Sort characters by frequency" (LeetCode 451).
• A variation of top-k frequent elements, applied to characters.

Clarifying questions to ask

1. How should ties be ordered?
2. Case-sensitive? Which character set?
3. Should equal characters be grouped together? (Yes.)

Follow-ups to expect

1. "How do you order by frequency efficiently?"
2. "Why is bucket sort linear?"
3. "How would you do it with a heap?"
4. "Complexity?"

Related questions

• Check if two strings are anagram of each other Easy
• Isomorphic String Easy
• Replace Elements by Their Rank Easy
• Roman to Integer Medium
• Sum of Beauty of All Substrings Medium
• Longest Repeating Character Replacement Hard

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