# Find Pivot Index

## Description

Given an array of integers `nums`, write a method that returns the "pivot" index of this array.

We define the pivot index as the index where the sum of the numbers to the left of the index is equal to the sum of the numbers to the right of the index.

If no such index exists, we should return -1. If there are multiple pivot indexes, you should return the left-most pivot index.

**Example 1:**<br>

```
Input: 
nums = [1, 7, 3, 6, 5, 6]
Output: 3
Explanation: 
The sum of the numbers to the left of index 3 (nums[3] = 6) is equal to the sum of numbers to the right of index 3.
Also, 3 is the first index where this occurs.
```

**Example 2:**<br>

```
Input: 
nums = [1, 2, 3]
Output: -1
Explanation: 
There is no index that satisfies the conditions in the problem statement.
```

**Note:**

* &#x20;The length of `nums` will be in the range `[0, 10000]`.
* &#x20;Each element `nums[i]` will be an integer in the range `[-1000, 1000]`.

## Solution

```cpp
class Solution {
public:
    int pivotIndex(vector<int>& nums) {
        int total_sum = accumulate(nums.begin(), nums.end(), 0);
        int sum = 0; // sum of first i numbers, i.e., nums[0:i], not including nums[i]
        for(int i = 0; i < nums.size(); ++i){
            // sum to the left == sum
            // sum to the right === total_sum - sum - nums[i]
            // check if sum == total_sum - sum - nums[i]
            // <==> 2 * sum + nums[i] == total_sum ?
            if(2 * sum + nums[i] == total_sum)
                return i;
            sum += nums[i];
        }
        return -1;
    }
};
```


---

# 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://twchen.gitbook.io/leetcode/array/find-pivot-index.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.