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feat: add new lc problems
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# [1990. Count the Number of Experiments](https://leetcode-cn.com/problems/count-the-number-of-experiments)
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[English Version](/solution/1900-1999/1990.Count%20the%20Number%20of%20Experiments/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>Experiments</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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| experiment_id | int |
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| platform | enum |
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| experiment_name | enum |
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+-----------------+------+
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experiment_id is the primary key for this table.
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platform is an enum with one of the values (&#39;Android&#39;, &#39;IOS&#39;, &#39;Web&#39;).
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experiment_name is an enum with one of the values (&#39;Reading&#39;, &#39;Sports&#39;, &#39;Programming&#39;).
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This table contains information about the ID of an experiment done with a random person, the platform used to do the experiment, and the name of the experiment.
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</pre>
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<p>&nbsp;</p>
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<p>Write an SQL query to report the <strong>number of experiments</strong> done on each of the three platforms for each of the three given experiments. Notice that all the pairs of (platform, experiment) should be included in the output <strong>including</strong> the pairs with <strong>zero experiments</strong>.</p>
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<p>Return the result table in <strong>any order</strong>.</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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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input:</strong>
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Experiments table:
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+---------------+----------+-----------------+
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| experiment_id | platform | experiment_name |
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+---------------+----------+-----------------+
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| 4 | IOS | Programming |
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| 13 | IOS | Sports |
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| 14 | Android | Reading |
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| 8 | Web | Reading |
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| 12 | Web | Reading |
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| 18 | Web | Programming |
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+---------------+----------+-----------------+
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<strong>Output:</strong>
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+----------+-----------------+-----------------+
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| platform | experiment_name | num_experiments |
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+----------+-----------------+-----------------+
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| Android | Reading | 1 |
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| Android | Sports | 0 |
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| Android | Programming | 0 |
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| IOS | Reading | 0 |
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| IOS | Sports | 1 |
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| IOS | Programming | 1 |
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| Web | Reading | 2 |
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| Web | Sports | 0 |
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| Web | Programming | 1 |
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+----------+-----------------+-----------------+
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<strong>Explanation:</strong>
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On the platform &quot;Android&quot;, we had only one &quot;Reading&quot; experiment.
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On the platform &quot;IOS&quot;, we had one &quot;Sports&quot; experiment and one &quot;Programming&quot; experiment.
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On the platform &quot;Web&quot;, we had two &quot;Reading&quot; experiments and one &quot;Programming&quot; experiment.
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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 -->
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# [1990. Count the Number of Experiments](https://leetcode.com/problems/count-the-number-of-experiments)
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[中文文档](/solution/1900-1999/1990.Count%20the%20Number%20of%20Experiments/README.md)
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## Description
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<p>Table: <code>Experiments</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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| experiment_id | int |
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| platform | enum |
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| experiment_name | enum |
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+-----------------+------+
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experiment_id is the primary key for this table.
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platform is an enum with one of the values (&#39;Android&#39;, &#39;IOS&#39;, &#39;Web&#39;).
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experiment_name is an enum with one of the values (&#39;Reading&#39;, &#39;Sports&#39;, &#39;Programming&#39;).
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This table contains information about the ID of an experiment done with a random person, the platform used to do the experiment, and the name of the experiment.
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</pre>
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<p>&nbsp;</p>
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<p>Write an SQL query to report the <strong>number of experiments</strong> done on each of the three platforms for each of the three given experiments. Notice that all the pairs of (platform, experiment) should be included in the output <strong>including</strong> the pairs with <strong>zero experiments</strong>.</p>
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<p>Return the result table in <strong>any order</strong>.</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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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input:</strong>
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Experiments table:
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+---------------+----------+-----------------+
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| experiment_id | platform | experiment_name |
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+---------------+----------+-----------------+
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| 4 | IOS | Programming |
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| 13 | IOS | Sports |
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| 14 | Android | Reading |
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| 8 | Web | Reading |
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| 12 | Web | Reading |
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| 18 | Web | Programming |
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+---------------+----------+-----------------+
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<strong>Output:</strong>
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+----------+-----------------+-----------------+
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| platform | experiment_name | num_experiments |
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+----------+-----------------+-----------------+
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| Android | Reading | 1 |
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| Android | Sports | 0 |
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| Android | Programming | 0 |
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| IOS | Reading | 0 |
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| IOS | Sports | 1 |
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| IOS | Programming | 1 |
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| Web | Reading | 2 |
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| Web | Sports | 0 |
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| Web | Programming | 1 |
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+----------+-----------------+-----------------+
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<strong>Explanation:</strong>
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On the platform &quot;Android&quot;, we had only one &quot;Reading&quot; experiment.
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On the platform &quot;IOS&quot;, we had one &quot;Sports&quot; experiment and one &quot;Programming&quot; experiment.
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On the platform &quot;Web&quot;, we had two &quot;Reading&quot; experiments and one &quot;Programming&quot; experiment.
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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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# [1991. 找到数组的中间位置](https://leetcode-cn.com/problems/find-the-middle-index-in-array)
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[English Version](/solution/1900-1999/1991.Find%20the%20Middle%20Index%20in%20Array/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>给你一个下标从 <strong>0</strong>&nbsp;开始的整数数组&nbsp;<code>nums</code>&nbsp;,请你找到 <strong>最左边</strong>&nbsp;的中间位置&nbsp;<code>middleIndex</code>&nbsp;(也就是所有可能中间位置下标最小的一个)。</p>
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<p>中间位置&nbsp;<code>middleIndex</code>&nbsp;是满足&nbsp;<code>nums[0] + nums[1] + ... + nums[middleIndex-1] == nums[middleIndex+1] + nums[middleIndex+2] + ... + nums[nums.length-1]</code>&nbsp;的数组下标。</p>
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<p>如果&nbsp;<code>middleIndex == 0</code>&nbsp;,左边部分的和定义为 <code>0</code>&nbsp;。类似的,如果&nbsp;<code>middleIndex == nums.length - 1</code>&nbsp;,右边部分的和定义为&nbsp;<code>0</code>&nbsp;。</p>
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<p>请你返回满足上述条件 <strong>最左边</strong>&nbsp;的<em>&nbsp;</em><code>middleIndex</code>&nbsp;,如果不存在这样的中间位置,请你返回&nbsp;<code>-1</code>&nbsp;。</p>
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<p>&nbsp;</p>
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<p><strong>示例 1:</strong></p>
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<pre><b>输入:</b>nums = [2,3,-1,<em><strong>8</strong></em>,4]
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<b>输出:</b>3
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<strong>解释:</strong>
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下标 3 之前的数字和为:2 + 3 + -1 = 4
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下标 3 之后的数字和为:4 = 4
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</pre>
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<p><strong>示例 2:</strong></p>
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<pre><b>输入:</b>nums = [1,-1,<em><strong>4</strong></em>]
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<b>输出:</b>2
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<strong>解释:</strong>
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下标 2 之前的数字和为:1 + -1 = 0
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下标 2 之后的数字和为:0
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</pre>
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<p><strong>示例 3:</strong></p>
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<pre><b>输入:</b>nums = [2,5]
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<b>输出:</b>-1
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<b>解释:</b>
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不存在符合要求的 middleIndex 。
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</pre>
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<p><strong>示例 4:</strong></p>
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<pre><b>输入:</b>nums = [<em><strong>1</strong></em>]
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<b>输出:</b>0
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<strong>解释:</strong>
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下标 0 之前的数字和为:0
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下标 0 之后的数字和为:0
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</pre>
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<p>&nbsp;</p>
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<p><strong>提示:</strong></p>
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<ul>
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<li><code>1 &lt;= nums.length &lt;= 100</code></li>
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<li><code>-1000 &lt;= nums[i] &lt;= 1000</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 -->
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# [1991. Find the Middle Index in Array](https://leetcode.com/problems/find-the-middle-index-in-array)
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[中文文档](/solution/1900-1999/1991.Find%20the%20Middle%20Index%20in%20Array/README.md)
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## Description
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<p>Given a <strong>0-indexed</strong> integer array <code>nums</code>, find the <strong>leftmost</strong> <code>middleIndex</code> (i.e., the smallest amongst all the possible ones).</p>
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<p>A <code>middleIndex</code> is an index where <code>nums[0] + nums[1] + ... + nums[middleIndex-1] == nums[middleIndex+1] + nums[middleIndex+2] + ... + nums[nums.length-1]</code>.</p>
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<p>If <code>middleIndex == 0</code>, the left side sum is considered to be <code>0</code>. Similarly, if <code>middleIndex == nums.length - 1</code>, the right side sum is considered to be <code>0</code>.</p>
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<p>Return <em>the <strong>leftmost</strong> </em><code>middleIndex</code><em> that satisfies the condition, or </em><code>-1</code><em> if there is no such index</em>.</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 = [2,3,-1,<u>8</u>,4]
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<strong>Output:</strong> 3
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<strong>Explanation:</strong>
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The sum of the numbers before index 3 is: 2 + 3 + -1 = 4
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The sum of the numbers after index 3 is: 4 = 4
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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 = [1,-1,<u>4</u>]
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<strong>Output:</strong> 2
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<strong>Explanation:</strong>
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The sum of the numbers before index 2 is: 1 + -1 = 0
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The sum of the numbers after index 2 is: 0
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</pre>
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<p><strong>Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> nums = [2,5]
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<strong>Output:</strong> -1
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<strong>Explanation:</strong>
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There is no valid middleIndex.
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</pre>
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<p><strong>Example 4:</strong></p>
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<pre>
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<strong>Input:</strong> nums = [<u>1</u>]
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<strong>Output:</strong> 0
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<strong>Explantion:</strong>
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The sum of the numbers before index 0 is: 0
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The sum of the numbers after index 0 is: 0
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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>1 &lt;= nums.length &lt;= 100</code></li>
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<li><code>-1000 &lt;= nums[i] &lt;= 1000</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 -->

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