Update

Author: ex01tus

README.md

@@ -2,14 +2,14 @@
 
 ![DSA Learning](https://img.shields.io/badge/DSA-Learning-blue?style=for-the-badge&logo=leetcode)
 
-This repository contains solutions for 544 DSA problems from the [LeetCode](https://leetcode.com/)
+This repository contains solutions for 573 DSA problems from the [LeetCode](https://leetcode.com/)
 website.
 
-Problems are separated into 19 topics:
+Problems are separated into 20 topics:
 
 |    | Topic               | Package                                        | Problems |
 |---:|:--------------------|:-----------------------------------------------|---------:|
-|  1 | Array               | [array](src/array)                             |       79 |
+|  1 | Array               | [array](src/array)                             |       80 |
 |  2 | Binary              | [binary](src/binary)                           |       13 |
 |  3 | Binary Search       | [binary_search](src/binary_search)             |       19 |
 |  4 | Binary Search Tree  | [binary_search_tree](src/binary_search_tree)   |       16 |
@@ -25,9 +25,10 @@ Problems are separated into 19 topics:
 | 14 | Matrix              | [matrix](src/matrix)                           |       18 |
 | 15 | Quad Tree           | [quad_tree](src/quad_tree)                     |        1 |
 | 16 | Recursion           | [recursion](src/recursion)                     |       24 |
-| 17 | Stack               | [stack](src/stack)                             |       25 |
-| 18 | String              | [string](src/string)                           |       56 |
-| 19 | Trie                | [trie](src/trie)                               |        4 |
+| 17 | SQL                 | [sql](src/sql)                                 |       28 |
+| 18 | Stack               | [stack](src/stack)                             |       25 |
+| 19 | String              | [string](src/string)                           |       56 |
+| 20 | Trie                | [trie](src/trie)                               |        4 |
 
 ## Good places to start your own LeetCode journey
 

src/array/SortArrayByIncreasingFrequency.java

@@ -0,0 +1,37 @@
+package array;
+
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Description: https://leetcode.com/problems/sort-array-by-increasing-frequency
+ * Difficulty: Easy
+ * Time complexity: O(n)
+ * Space complexity: O(n)
+ */
+public class SortArrayByIncreasingFrequency {
+
+    public int[] frequencySort(int[] nums) {
+        Map<Integer, Integer> freqMap = buildFreqMap(nums);
+        return Arrays.stream(nums)
+                .boxed()
+                .sorted((a, b) -> compare(a, b, freqMap))
+                .mapToInt(v -> v)
+                .toArray();
+    }
+
+    private Map<Integer, Integer> buildFreqMap(int[] nums) {
+        Map<Integer, Integer> freqMap = new HashMap<>();
+        for (int num : nums) {
+            freqMap.merge(num, 1, Integer::sum);
+        }
+
+        return freqMap;
+    }
+
+    private int compare(int a, int b, Map<Integer, Integer> freqMap) {
+        int result = Integer.compare(freqMap.get(a), freqMap.get(b));
+        return result != 0 ? result : Integer.compare(b, a);
+    }
+}

src/sql/ArticleViews.sql

@@ -0,0 +1,7 @@
+-- Description: https://leetcode.com/problems/article-views-i
+-- Difficulty: Easy
+
+SELECT DISTINCT author_id AS id
+FROM Views
+WHERE author_id = viewer_id
+ORDER BY author_id;

src/sql/AverageSellingPrice.sql

@@ -0,0 +1,10 @@
+-- Description: https://leetcode.com/problems/average-selling-price
+-- Difficulty: Easy
+
+SELECT s.product_id,
+       ROUND(SUM(s.units * p.price) / SUM(s.units), 2) AS average_price
+FROM UnitsSold AS s
+         LEFT JOIN Prices AS p
+                   ON s.product_id = p.product_id
+                       AND s.purchase_date BETWEEN p.start_date AND p.end_date
+GROUP BY s.product_id;

src/sql/AverageTimeOfProcessPerMachine.sql

@@ -0,0 +1,11 @@
+-- Description: https://leetcode.com/problems/average-time-of-process-per-machine
+-- Difficulty: Easy
+
+SELECT s.machine_id, ROUND(AVG(e.timestamp - s.timestamp), 3) AS processing_time
+FROM Activity AS s
+         INNER JOIN Activity AS e
+                    ON s.machine_id = e.machine_id
+                        AND s.process_id = e.process_id
+                        AND s.activity_type = 'start'
+                        AND e.activity_type = 'end'
+GROUP BY s.machine_id;

src/sql/BigCountries.sql

@@ -0,0 +1,7 @@
+-- Description: https://leetcode.com/problems/big-countries
+-- Difficulty: Easy
+
+SELECT name, population, area
+FROM World
+WHERE area >= 3000000
+   OR population >= 25000000;

src/sql/BiggestSingleNumber.sql

@@ -0,0 +1,8 @@
+-- Description: https://leetcode.com/problems/biggest-single-number
+-- Difficulty: Easy
+
+SELECT MAX(num) AS num
+FROM (SELECT num
+      FROM MyNumbers
+      GROUP BY num
+      HAVING COUNT(num) = 1) AS tmp;

src/sql/ClassesWithMoreThanFiveStudents.sql

@@ -0,0 +1,7 @@
+-- Description: https://leetcode.com/problems/classes-more-than-5-students
+-- Difficulty: Easy
+
+SELECT class
+FROM Courses
+GROUP BY class
+HAVING COUNT(DISTINCT student) >= 5;

src/sql/ConfirmationRate.sql

@@ -0,0 +1,9 @@
+-- Description: https://leetcode.com/problems/confirmation-rate
+-- Difficulty: Medium
+
+SELECT s.user_id,
+       ROUND(AVG(CASE WHEN c.action = 'confirmed' THEN 1.0 ELSE 0.0 END), 2) AS confirmation_rate
+FROM Signups AS s
+         LEFT JOIN Confirmations AS c
+                   ON s.user_id = c.user_id
+GROUP BY s.user_id

src/sql/CustomerWhoVisitedButDidntMakeAnyTransactions.sql

@@ -0,0 +1,9 @@
+-- Description: https://leetcode.com/problems/customer-who-visited-but-did-not-make-any-transactions
+-- Difficulty: Easy
+
+SELECT v.customer_id, COUNT(v.visit_id) AS count_no_trans
+FROM Visits AS v
+         LEFT JOIN Transactions AS t
+                   ON v.visit_id = t.visit_id
+WHERE t.transaction_id IS NULL
+GROUP BY v.customer_id;