# 1991. Find the Middle Index in Array

<https://leetcode.com/problems/find-the-middle-index-in-array>

## Description

Given a **0-indexed** integer array `nums`, find the **leftmost** `middleIndex` (i.e., the smallest amongst all the possible ones).

A `middleIndex` is an index where `nums[0] + nums[1] + ... + nums[middleIndex-1] == nums[middleIndex+1] + nums[middleIndex+2] + ... + nums[nums.length-1]`.

If `middleIndex == 0`, the left side sum is considered to be `0`. Similarly, if `middleIndex == nums.length - 1`, the right side sum is considered to be `0`.

Return *the **leftmost*** `middleIndex` *that satisfies the condition, or* `-1` *if there is no such index*.

**Example 1:**

```
**Input:** nums = [2,3,-1,8,4]
**Output:** 3
**Explanation:**
The sum of the numbers before index 3 is: 2 + 3 + -1 = 4
The sum of the numbers after index 3 is: 4 = 4
```

**Example 2:**

```
**Input:** nums = [1,-1,4]
**Output:** 2
**Explanation:**
The sum of the numbers before index 2 is: 1 + -1 = 0
The sum of the numbers after index 2 is: 0
```

**Example 3:**

```
**Input:** nums = [2,5]
**Output:** -1
**Explanation:**
There is no valid middleIndex.
```

**Example 4:**

```
**Input:** nums = [1]
**Output:** 0
**Explantion:**
The sum of the numbers before index 0 is: 0
The sum of the numbers after index 0 is: 0
```

**Constraints:**

* `1 <= nums.length <= 100`
* `-1000 <= nums[i] <= 1000`

**Note:** This question is the same as 724: <https://leetcode.com/problems/find-pivot-index/>

## ac

```java
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://jaywin.gitbook.io/leetcode/solutions/1991-find-the-middle-index-in-array.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.