724. Find Pivot Index

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Given an array of integers `nums`

, calculate the **pivot index** of this array.

The **pivot index** is the index where the sum of all the numbers **strictly** to the left of the index is equal to the sum of all the numbers **strictly** to the index's right.

If the index is on the left edge of the array, then the left sum is `0`

because there are no elements to the left. This also applies to the right edge of the array.

Return *the leftmost pivot index*. If no such index exists, return

`-1`

.

**Example 1:**

Input:nums = [1,7,3,6,5,6]Output:3Explanation:The pivot index is 3. Left sum = nums[0] + nums[1] + nums[2] = 1 + 7 + 3 = 11 Right sum = nums[4] + nums[5] = 5 + 6 = 11

**Example 2:**

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

**Example 3:**

Input:nums = [2,1,-1]Output:0Explanation:The pivot index is 0. Left sum = 0 (no elements to the left of index 0) Right sum = nums[1] + nums[2] = 1 + -1 = 0

**Constraints:**

`1 <= nums.length <= 10`

^{4}`-1000 <= nums[i] <= 1000`

**Note:** This question is the same as 1991: https://leetcode.com/problems/find-the-middle-index-in-array/

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