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feat: add solutions to lc problems: No.1918,1919,1920,1921
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‎solution/1900-1999/1918.Kth Smallest Subarray Sum/README.md

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# [1918. Kth Smallest Subarray Sum](https://leetcode-cn.com/problems/kth-smallest-subarray-sum)
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[English Version](/solution/1900-1999/1918.Kth%20Smallest%20Subarray%20Sum/README_EN.md)
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## 题目描述
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<!-- 这里写题目描述 -->
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<p>Given an integer array <code>nums</code> of length <code>n</code> and an integer <code>k</code>, return <em>the </em><code>k<sup>th</sup></code> <em><strong>smallest subarray sum</strong>.</em></p>
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<p>A <strong>subarray</strong> is defined as a <strong>non-empty</strong> contiguous sequence of elements in an array. A <strong>subarray sum</strong> is the sum of all elements in the subarray.</p>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> nums = [2,1,3], k = 4
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<strong>Output:</strong> 3
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<strong>Explanation: </strong>The subarrays of [2,1,3] are:
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- [2] with sum 2
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- [1] with sum 1
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- [3] with sum 3
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- [2,1] with sum 3
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- [1,3] with sum 4
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- [2,1,3] with sum 6
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Ordering the sums from smallest to largest gives 1, 2, 3, <u>3</u>, 4, 6. The 4th smallest is 3.
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</pre>
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<p><strong>Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> nums = [3,3,5,5], k = 7
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<strong>Output:</strong> 10
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<strong>Explanation: </strong>The subarrays of [3,3,5,5] are:
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- [3] with sum 3
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- [3] with sum 3
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- [5] with sum 5
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- [5] with sum 5
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- [3,3] with sum 6
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- [3,5] with sum 8
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- [5,5] with sum 10
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- [3,3,5], with sum 11
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- [3,5,5] with sum 13
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- [3,3,5,5] with sum 16
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Ordering the sums from smallest to largest gives 3, 3, 5, 5, 6, 8, <u>10</u>, 11, 13, 16. The 7th smallest is 10.
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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>n == nums.length</code></li>
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<li><code>1 &lt;= n&nbsp;&lt;= 2 * 10<sup>4</sup></code></li>
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<li><code>1 &lt;= nums[i] &lt;= 5 * 10<sup>4</sup></code></li>
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<li><code>1 &lt;= k &lt;= n * (n + 1) / 2</code></li>
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</ul>
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## 解法
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<!-- 这里可写通用的实现逻辑 -->
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<!-- tabs:start -->
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### **Python3**
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<!-- 这里可写当前语言的特殊实现逻辑 -->
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```python
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```
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### **Java**
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<!-- 这里可写当前语言的特殊实现逻辑 -->
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```java
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```
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### **...**
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```
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```
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<!-- tabs:end -->

‎solution/1900-1999/1918.Kth Smallest Subarray Sum/README_EN.md

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# [1918. Kth Smallest Subarray Sum](https://leetcode.com/problems/kth-smallest-subarray-sum)
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[中文文档](/solution/1900-1999/1918.Kth%20Smallest%20Subarray%20Sum/README.md)
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## Description
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<p>Given an integer array <code>nums</code> of length <code>n</code> and an integer <code>k</code>, return <em>the </em><code>k<sup>th</sup></code> <em><strong>smallest subarray sum</strong>.</em></p>
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<p>A <strong>subarray</strong> is defined as a <strong>non-empty</strong> contiguous sequence of elements in an array. A <strong>subarray sum</strong> is the sum of all elements in the subarray.</p>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> nums = [2,1,3], k = 4
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<strong>Output:</strong> 3
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<strong>Explanation: </strong>The subarrays of [2,1,3] are:
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- [2] with sum 2
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- [1] with sum 1
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- [3] with sum 3
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- [2,1] with sum 3
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- [1,3] with sum 4
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- [2,1,3] with sum 6
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Ordering the sums from smallest to largest gives 1, 2, 3, <u>3</u>, 4, 6. The 4th smallest is 3.
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</pre>
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<p><strong>Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> nums = [3,3,5,5], k = 7
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<strong>Output:</strong> 10
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<strong>Explanation: </strong>The subarrays of [3,3,5,5] are:
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- [3] with sum 3
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- [3] with sum 3
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- [5] with sum 5
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- [5] with sum 5
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- [3,3] with sum 6
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- [3,5] with sum 8
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- [5,5] with sum 10
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- [3,3,5], with sum 11
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- [3,5,5] with sum 13
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- [3,3,5,5] with sum 16
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Ordering the sums from smallest to largest gives 3, 3, 5, 5, 6, 8, <u>10</u>, 11, 13, 16. The 7th smallest is 10.
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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>n == nums.length</code></li>
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<li><code>1 &lt;= n&nbsp;&lt;= 2 * 10<sup>4</sup></code></li>
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<li><code>1 &lt;= nums[i] &lt;= 5 * 10<sup>4</sup></code></li>
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<li><code>1 &lt;= k &lt;= n * (n + 1) / 2</code></li>
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</ul>
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## Solutions
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<!-- tabs:start -->
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### **Python3**
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```python
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```
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### **Java**
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```java
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```
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### **...**
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```
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```
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<!-- tabs:end -->

‎solution/1900-1999/1919.Leetcodify Similar Friends/README.md

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# [1919. Leetcodify Similar Friends](https://leetcode-cn.com/problems/leetcodify-similar-friends)
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[English Version](/solution/1900-1999/1919.Leetcodify%20Similar%20Friends/README_EN.md)
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## 题目描述
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<!-- 这里写题目描述 -->
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<p>Table: <code>Listens</code></p>
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<pre>
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+-------------+---------+
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| Column Name | Type |
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+-------------+---------+
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| user_id | int |
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| song_id | int |
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| day | date |
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+-------------+---------+
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There is no primary key for this table. It may contain duplicates.
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Each row of this table indicates that the user user_id listened to the song song_id on the day day.
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</pre>
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<p>&nbsp;</p>
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<p>Table: <code>Friendship</code></p>
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<pre>
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+---------------+---------+
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| Column Name | Type |
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+---------------+---------+
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| user1_id | int |
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| user2_id | int |
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+---------------+---------+
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(user1_id, user2_id) is the primary key for this table.
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Each row of this table indicates that the users user1_id and user2_id are friends.
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Note that user1_id &lt; user2_id.
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</pre>
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<p>&nbsp;</p>
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<p>Write an SQL query to report the similar friends of Leetcodify users. A user <code>x</code> and user <code>y</code> are&nbsp;similar friends if:</p>
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<ul>
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<li>Users <code>x</code> and <code>y</code> are friends, and</li>
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<li>Users <code>x</code> and <code>y</code> listened to the same three or more different songs <strong>on the same day</strong>.</li>
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</ul>
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<p>Return the result table in <strong>any order</strong>. Note that you must return the similar pairs of friends the same way they were represented in the input (i.e., always <code>user1_id &lt; user2_id</code>).</p>
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<p>The query result format is in the following example:</p>
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<p>&nbsp;</p>
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<pre>
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Listens table:
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+---------+---------+------------+
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| user_id | song_id | day |
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+---------+---------+------------+
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| 1 | 10 | 2021-03-15 |
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| 1 | 11 | 2021-03-15 |
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| 1 | 12 | 2021-03-15 |
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| 2 | 10 | 2021-03-15 |
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| 2 | 11 | 2021-03-15 |
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| 2 | 12 | 2021-03-15 |
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| 3 | 10 | 2021-03-15 |
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| 3 | 11 | 2021-03-15 |
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| 3 | 12 | 2021-03-15 |
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| 4 | 10 | 2021-03-15 |
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| 4 | 11 | 2021-03-15 |
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| 4 | 13 | 2021-03-15 |
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| 5 | 10 | 2021-03-16 |
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| 5 | 11 | 2021-03-16 |
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| 5 | 12 | 2021-03-16 |
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+---------+---------+------------+
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Friendship table:
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+----------+----------+
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| user1_id | user2_id |
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+----------+----------+
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| 1 | 2 |
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| 2 | 4 |
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| 2 | 5 |
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+----------+----------+
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Result table:
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+----------+----------+
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| user1_id | user2_id |
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+----------+----------+
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| 1 | 2 |
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+----------+----------+
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Users 1 and 2 are friends, and they listened to songs 10, 11, and 12 on the same day. They are similar friends.
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Users 1 and 3 listened to songs 10, 11, and 12 on the same day, but they are not friends.
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Users 2 and 4 are friends, but they did not listen to the same three different songs.
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Users 2 and 5 are friends and listened to songs 10, 11, and 12, but they did not listen to them on the same day.
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</pre>
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## 解法
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<!-- 这里可写通用的实现逻辑 -->
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<!-- tabs:start -->
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### **SQL**
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<!-- 这里可写当前语言的特殊实现逻辑 -->
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```sql
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```
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<!-- tabs:end -->

‎solution/1900-1999/1919.Leetcodify Similar Friends/README_EN.md

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# [1919. Leetcodify Similar Friends](https://leetcode.com/problems/leetcodify-similar-friends)
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[中文文档](/solution/1900-1999/1919.Leetcodify%20Similar%20Friends/README.md)
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## Description
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<p>Table: <code>Listens</code></p>
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<pre>
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+-------------+---------+
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| Column Name | Type |
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+-------------+---------+
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| user_id | int |
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| song_id | int |
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| day | date |
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+-------------+---------+
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There is no primary key for this table. It may contain duplicates.
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Each row of this table indicates that the user user_id listened to the song song_id on the day day.
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</pre>
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<p>&nbsp;</p>
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<p>Table: <code>Friendship</code></p>
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<pre>
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+---------------+---------+
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| Column Name | Type |
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+---------------+---------+
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| user1_id | int |
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| user2_id | int |
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+---------------+---------+
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(user1_id, user2_id) is the primary key for this table.
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Each row of this table indicates that the users user1_id and user2_id are friends.
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Note that user1_id &lt; user2_id.
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</pre>
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<p>&nbsp;</p>
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<p>Write an SQL query to report the similar friends of Leetcodify users. A user <code>x</code> and user <code>y</code> are&nbsp;similar friends if:</p>
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<ul>
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<li>Users <code>x</code> and <code>y</code> are friends, and</li>
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<li>Users <code>x</code> and <code>y</code> listened to the same three or more different songs <strong>on the same day</strong>.</li>
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</ul>
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<p>Return the result table in <strong>any order</strong>. Note that you must return the similar pairs of friends the same way they were represented in the input (i.e., always <code>user1_id &lt; user2_id</code>).</p>
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<p>The query result format is in the following example:</p>
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<p>&nbsp;</p>
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<pre>
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Listens table:
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+---------+---------+------------+
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| user_id | song_id | day |
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+---------+---------+------------+
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| 1 | 10 | 2021-03-15 |
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| 1 | 11 | 2021-03-15 |
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| 1 | 12 | 2021-03-15 |
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| 2 | 10 | 2021-03-15 |
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| 2 | 11 | 2021-03-15 |
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| 2 | 12 | 2021-03-15 |
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| 3 | 10 | 2021-03-15 |
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| 3 | 11 | 2021-03-15 |
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| 3 | 12 | 2021-03-15 |
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| 4 | 10 | 2021-03-15 |
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| 4 | 11 | 2021-03-15 |
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| 4 | 13 | 2021-03-15 |
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| 5 | 10 | 2021-03-16 |
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| 5 | 11 | 2021-03-16 |
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| 5 | 12 | 2021-03-16 |
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+---------+---------+------------+
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Friendship table:
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+----------+----------+
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| user1_id | user2_id |
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+----------+----------+
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| 1 | 2 |
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| 2 | 4 |
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| 2 | 5 |
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+----------+----------+
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Result table:
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+----------+----------+
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| user1_id | user2_id |
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+----------+----------+
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| 1 | 2 |
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+----------+----------+
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Users 1 and 2 are friends, and they listened to songs 10, 11, and 12 on the same day. They are similar friends.
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Users 1 and 3 listened to songs 10, 11, and 12 on the same day, but they are not friends.
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Users 2 and 4 are friends, but they did not listen to the same three different songs.
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Users 2 and 5 are friends and listened to songs 10, 11, and 12, but they did not listen to them on the same day.
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</pre>
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## Solutions
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<!-- tabs:start -->
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### **SQL**
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```sql
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```
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<!-- tabs:end -->

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