[REFACTOR] [Hacker Rank] Interview Preparation Kit: Dictionaries and Hashmaps: Frequency Queries. New solution notes document added.

Author: Gonzalo Diaz

docs/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/frequency-queries-solution-notes.md

@@ -0,0 +1,27 @@
+# [Dictionaries and Hashmaps: Frequency Queries](https://www.hackerrank.com/challenges/frequency-queries)
+
+- Difficulty:  `#medium`
+- Category: `#ProblemSolvingIntermediate`  `#DictionariesAndHashmaps`
+
+## First solution
+
+The first solution is based on the idea of ​​storing the values
+​​of insert and delete operations in a dictionary.
+
+For the "select" operation, a search loop is made to find the expected frequency,
+which in the best case is cut off when the value is found,
+but in the worst case it goes through the entire dictionary.
+
+## Optimized solution
+
+The optimized solution tries to reduce the worst case problem above,
+reducing the select operation to direct access to the expected frequency.
+
+To achieve this, it is necessary to maintain an "inverted" dictionary
+where the frequency values ​​are the keys and the values ​​of each element
+are stored in a list of values ​​that have the same frequency.
+
+To maintain this structure, any operation that alters the data (insert, delete),
+must update "in which frequency group" the element will be.
+
+This solution transfers the cost of the search to the data update operations.

docs/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/frequency-queries.md

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 # [Dictionaries and Hashmaps: Frequency Queries](https://www.hackerrank.com/challenges/frequency-queries)
 
 - Difficulty:  `#medium`
-- Category: `#ProblemSolvingIntermediate`
+- Category: `#ProblemSolvingIntermediate`  `#DictionariesAndHashmaps`
 
 You are given  queries. Each query is of the form two integers described below:
 

src/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/frequency_queries_bruteforce.js

@@ -1,5 +1,6 @@
 /**
  * @link Problem definition [[docs/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/frequency-queries.md]]
+ * @see Solution Notes: [[docs/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/frequency-queries-solution-notes.md]]
  */
 
 export function freqQuery(queries) {

src/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/frequency_queries_optimized.js

@@ -1,5 +1,6 @@
 /**
  * @link Problem definition [[docs/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/frequency-queries.md]]
+ * @see Solution Notes: [[docs/hackerrank/interview_preparation_kit/dictionaries_and_hashmaps/frequency-queries-solution-notes.md]]
  */
 
 export function updateFrequency(frequencyMap, data, currentFreq, newFreq) {