1228. Missing Number In Arithmetic Progression

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 .

Then, a value from arr  was removed that was not the first or last value in the array .

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] <= 10^5