Is Sort Characters By Frequency easy or hard?

Sort Characters By Frequency is generally considered a medium-level problem. The core challenge is recognizing the frequency-count pattern and selecting the right structure to order characters efficiently, such as sorting, a heap, or bucket sort.

Sort Characters By Frequency Python/Java solution

In Python or Java, the common solution counts characters with a dictionary or HashMap, then sorts the entries by frequency in descending order and builds the output string. Python often uses collections.Counter with sorted, while Java uses HashMap with a custom comparator or priority queue.

How to solve Sort Characters By Frequency in O(n)?

Use a frequency map to count occurrences of each character. Create an array of buckets where index i stores characters that appear i times. Iterate from the highest frequency bucket down to 1 and append characters accordingly. Since each character is processed a constant number of times, the overall complexity is O(n).

What is the best approach for Sort Characters By Frequency?

The most practical solution uses a HashMap to count character frequencies and then sorts the characters by frequency in descending order. This runs in O(n log k) time where k is the number of unique characters. An even faster approach uses bucket sort to achieve O(n) time by grouping characters by their frequency.

Is Sort Characters By Frequency asked at Google/Amazon/Meta?

Frequency counting and string reordering problems appear frequently in interviews at companies like Amazon, Google, and Meta. Variations often test knowledge of hash tables, heaps, and bucket sort techniques for efficiently ordering elements by frequency.

What data structure is used in Sort Characters By Frequency?

The core data structure is a HashMap (or frequency array) used to count character occurrences. Depending on the solution, the ordering step uses sorting, a max heap (priority queue), or bucket sort to arrange characters by decreasing frequency.

What is the time complexity of Sort Characters By Frequency?

The HashMap plus sorting approach runs in O(n log k) time, where n is the string length and k is the number of distinct characters. Using bucket sort reduces the complexity to O(n) because characters are placed into frequency-indexed buckets instead of being comparison-sorted.

Description Solution Video 7:15

451. Sort Characters By Frequency

Medium75.8% acceptance16 companies

Given a string s, sort it in decreasing order based on the frequency of the characters. The frequency of a character is the number of times it appears in the string.

Return the sorted string. If there are multiple answers, return any of them.

Example 1:

Input: s = "tree"
Output: "eert"
Explanation: 'e' appears twice while 'r' and 't' both appear once.
So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.

Example 2:

Input: s = "cccaaa"
Output: "aaaccc"
Explanation: Both 'c' and 'a' appear three times, so both "cccaaa" and "aaaccc" are valid answers.
Note that "cacaca" is incorrect, as the same characters must be together.

Example 3:

Input: s = "Aabb"
Output: "bbAa"
Explanation: "bbaA" is also a valid answer, but "Aabb" is incorrect.
Note that 'A' and 'a' are treated as two different characters.

Constraints: