1228. Missing Number In Arithmetic Progression

https://leetcode.com/problems/missing-number-in-arithmetic-progression

Description

In some array arr, the values were in arithmetic progression: the values arr[i + 1] - arr[i] are all equal for every 0 <= i < arr.length - 1.

A value from arr was removed that was not the first or last value in the array.

Given arr, return the removed value.

Example 1:

**Input:** arr = [5,7,11,13]
**Output:** 9
**Explanation:** The previous array was [5,7,**9**,11,13].

Example 2:

**Input:** arr = [15,13,12]
**Output:** 14
**Explanation:** The previous array was [15,**14**,13,12].

Constraints:

• 3 <= arr.length <= 1000

• 0 <= arr[i] <= 105

• The given array is guaranteed to be a valid array.

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