chore: add new lc problems

solution/2800-2899/2859.Sum of Values at Indices With K Set Bits/README.md

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+# [2859. 计算 K 置位下标对应元素的和](https://leetcode.cn/problems/sum-of-values-at-indices-with-k-set-bits)
+
+[English Version](/solution/2800-2899/2859.Sum%20of%20Values%20at%20Indices%20With%20K%20Set%20Bits/README_EN.md)
+
+## 题目描述
+
+<!-- 这里写题目描述 -->
+
+<p>给你一个下标从 <strong>0</strong> 开始的整数数组 <code>nums</code> 和一个整数 <code>k</code> 。</p>
+
+<p>请你用整数形式返回 <code>nums</code> 中的特定元素之 <strong>和</strong> ，这些特定元素满足：其对应下标的二进制表示中恰存在 <code>k</code> 个置位。</p>
+
+<p>整数的二进制表示中的 1 就是这个整数的 <strong>置位</strong> 。</p>
+
+<p>例如，<code>21</code> 的二进制表示为 <code>10101</code> ，其中有 <code>3</code> 个置位。</p>
+
+<p>&nbsp;</p>
+
+<p><strong class="example">示例 1：</strong></p>
+
+<pre>
+<strong>输入：</strong>nums = [5,10,1,5,2], k = 1
+<strong>输出：</strong>13
+<strong>解释：</strong>下标的二进制表示是： 
+0 = 000<sub>2</sub>
+1 = 001<sub>2</sub>
+2 = 010<sub>2</sub>
+3 = 011<sub>2</sub>
+4 = 100<sub>2 
+</sub>下标 1、2 和 4 在其二进制表示中都存在 k = 1 个置位。
+因此，答案为 nums[1] + nums[2] + nums[4] = 13 。</pre>
+
+<p><strong class="example">示例 2：</strong></p>
+
+<pre>
+<strong>输入：</strong>nums = [4,3,2,1], k = 2
+<strong>输出：</strong>1
+<strong>解释：</strong>下标的二进制表示是： 
+0 = 00<sub>2</sub>
+1 = 01<sub>2</sub>
+2 = 10<sub>2</sub>
+3 = 11<sub>2
+</sub>只有下标 3 的二进制表示中存在 k = 2 个置位。
+因此，答案为 nums[3] = 1 。
+</pre>
+
+<p>&nbsp;</p>
+
+<p><strong>提示：</strong></p>
+
+<ul>
+	<li><code>1 &lt;= nums.length &lt;= 1000</code></li>
+	<li><code>1 &lt;= nums[i] &lt;= 10<sup>5</sup></code></li>
+	<li><code>0 &lt;= k &lt;= 10</code></li>
+</ul>
+
+## 解法
+
+<!-- 这里可写通用的实现逻辑 -->
+
+<!-- tabs:start -->
+
+### **Python3**
+
+<!-- 这里可写当前语言的特殊实现逻辑 -->
+
+```python
+
+```
+
+### **Java**
+
+<!-- 这里可写当前语言的特殊实现逻辑 -->
+
+```java
+
+```
+
+### **C++**
+
+```cpp
+
+```
+
+### **Go**
+
+```go
+
+```
+
+### **...**
+
+```
+
+```
+
+<!-- tabs:end -->

solution/2800-2899/2859.Sum of Values at Indices With K Set Bits/README_EN.md

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+# [2859. Sum of Values at Indices With K Set Bits](https://leetcode.com/problems/sum-of-values-at-indices-with-k-set-bits)
+
+[中文文档](/solution/2800-2899/2859.Sum%20of%20Values%20at%20Indices%20With%20K%20Set%20Bits/README.md)
+
+## Description
+
+<p>You are given a <strong>0-indexed</strong> integer array <code>nums</code> and an integer <code>k</code>.</p>
+
+<p>Return <em>an integer that denotes the <strong>sum</strong> of elements in </em><code>nums</code><em> whose corresponding <strong>indices</strong> have <strong>exactly</strong> </em><code>k</code><em> set bits in their binary representation.</em></p>
+
+<p>The <strong>set bits</strong> in an integer are the <code>1</code>&#39;s present when it is written in binary.</p>
+
+<ul>
+	<li>For example, the binary representation of <code>21</code> is <code>10101</code>, which has <code>3</code> set bits.</li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> nums = [5,10,1,5,2], k = 1
+<strong>Output:</strong> 13
+<strong>Explanation:</strong> The binary representation of the indices are: 
+0 = 000<sub>2</sub>
+1 = 001<sub>2</sub>
+2 = 010<sub>2</sub>
+3 = 011<sub>2</sub>
+4 = 100<sub>2 
+</sub>Indices 1, 2, and 4 have k = 1 set bits in their binary representation.
+Hence, the answer is nums[1] + nums[2] + nums[4] = 13.</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> nums = [4,3,2,1], k = 2
+<strong>Output:</strong> 1
+<strong>Explanation:</strong> The binary representation of the indices are:
+0 = 00<sub>2</sub>
+1 = 01<sub>2</sub>
+2 = 10<sub>2</sub>
+3 = 11<sub>2
+</sub>Only index 3 has k = 2 set bits in its binary representation.
+Hence, the answer is nums[3] = 1.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= nums.length &lt;= 1000</code></li>
+	<li><code>1 &lt;= nums[i] &lt;= 10<sup>5</sup></code></li>
+	<li><code>0 &lt;= k &lt;= 10</code></li>
+</ul>
+
+## Solutions
+
+<!-- tabs:start -->
+
+### **Python3**
+
+```python
+
+```
+
+### **Java**
+
+```java
+
+```
+
+### **C++**
+
+```cpp
+
+```
+
+### **Go**
+
+```go
+
+```
+
+### **...**
+
+```
+
+```
+
+<!-- tabs:end -->

solution/2800-2899/2860.Happy Students/README.md

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+# [2860. 让所有学生保持开心的分组方法数](https://leetcode.cn/problems/happy-students)
+
+[English Version](/solution/2800-2899/2860.Happy%20Students/README_EN.md)
+
+## 题目描述
+
+<!-- 这里写题目描述 -->
+
+<p>给你一个下标从 <strong>0</strong> 开始、长度为 <code>n</code> 的整数数组 <code>nums</code> ，其中 <code>n</code> 是班级中学生的总数。班主任希望能够在让所有学生保持开心的情况下选出一组学生：</p>
+
+<p>如果能够满足下述两个条件之一，则认为第 <code>i</code> 位学生将会保持开心：</p>
+
+<ul>
+	<li>这位学生被选中，并且被选中的学生人数 <strong>严格大于</strong> <code>nums[i]</code> 。</li>
+	<li>这位学生没有被选中，并且被选中的学生人数 <strong>严格小于</strong> <code>nums[i]</code> 。</li>
+</ul>
+
+<p>返回能够满足让所有学生保持开心的分组方法的数目。</p>
+
+<p>&nbsp;</p>
+
+<p><strong class="example">示例 1：</strong></p>
+
+<pre>
+<strong>输入：</strong>nums = [1,1]
+<strong>输出：</strong>2
+<strong>解释：</strong>
+有两种可行的方法：
+班主任没有选中学生。
+班主任选中所有学生形成一组。 
+如果班主任仅选中一个学生来完成分组，那么两个学生都无法保持开心。因此，仅存在两种可行的方法。
+</pre>
+
+<p><strong class="example">示例 2：</strong></p>
+
+<pre>
+<strong>输入：</strong>nums = [6,0,3,3,6,7,2,7]
+<strong>输出：</strong>3
+<strong>解释：</strong>
+存在三种可行的方法：
+班主任选中下标为 1 的学生形成一组。
+班主任选中下标为 1、2、3、6 的学生形成一组。
+班主任选中所有学生形成一组。 
+</pre>
+
+<p>&nbsp;</p>
+
+<p><strong>提示：</strong></p>
+
+<ul>
+	<li><code>1 &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
+	<li><code>0 &lt;= nums[i] &lt; nums.length</code></li>
+</ul>
+
+## 解法
+
+<!-- 这里可写通用的实现逻辑 -->
+
+<!-- tabs:start -->
+
+### **Python3**
+
+<!-- 这里可写当前语言的特殊实现逻辑 -->
+
+```python
+
+```
+
+### **Java**
+
+<!-- 这里可写当前语言的特殊实现逻辑 -->
+
+```java
+
+```
+
+### **C++**
+
+```cpp
+
+```
+
+### **Go**
+
+```go
+
+```
+
+### **...**
+
+```
+
+```
+
+<!-- tabs:end -->

solution/2800-2899/2860.Happy Students/README_EN.md

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+# [2860. Happy Students](https://leetcode.com/problems/happy-students)
+
+[中文文档](/solution/2800-2899/2860.Happy%20Students/README.md)
+
+## Description
+
+<p>You are given a <strong>0-indexed</strong> integer array <code>nums</code> of length <code>n</code> where <code>n</code> is the total number of students in the class. The class teacher tries to select a group of students so that all the students remain happy.</p>
+
+<p>The <code>i<sup>th</sup></code> student will become happy if one of these two conditions is met:</p>
+
+<ul>
+	<li>The student is selected and the total number of selected students is<strong> strictly greater than</strong> <code>nums[i]</code>.</li>
+	<li>The student is not selected and the total number of selected students is <strong>strictly</strong> <strong>less than</strong> <code>nums[i]</code>.</li>
+</ul>
+
+<p>Return <em>the number of ways to select a group of students so that everyone remains happy.</em></p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> nums = [1,1]
+<strong>Output:</strong> 2
+<strong>Explanation:</strong> 
+The two possible ways are:
+The class teacher selects no student.
+The class teacher selects both students to form the group. 
+If the class teacher selects just one student to form a group then the both students will not be happy. Therefore, there are only two possible ways.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> nums = [6,0,3,3,6,7,2,7]
+<strong>Output:</strong> 3
+<strong>Explanation:</strong> 
+The three possible ways are:
+The class teacher selects the student with index = 1 to form the group.
+The class teacher selects the students with index = 1, 2, 3, 6 to form the group.
+The class teacher selects all the students to form the group.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
+	<li><code>0 &lt;= nums[i] &lt; nums.length</code></li>
+</ul>
+
+## Solutions
+
+<!-- tabs:start -->
+
+### **Python3**
+
+```python
+
+```
+
+### **Java**
+
+```java
+
+```
+
+### **C++**
+
+```cpp
+
+```
+
+### **Go**
+
+```go
+
+```
+
+### **...**
+
+```
+
+```
+
+<!-- tabs:end -->