9

Missing number in Arithmetic progression

Difficulty: EASY
Avg. time to solve
15 min
Success Rate
85%

Problem Statement
Suggest Edit

You are given a sorted array of ‘N’ distinct integers that are in the Arithmetic Progression sequence except for one element which is missing from the sequence. You have to find that missing number from the given sequence.

Note:
1. A sequence [arr0, arr1,…, arr(n-1)] is called an Arithmetic progression if for each 'i' ( 0 ≤ i < n - 1) the value arr[i+1] − arr[i] is the same. 
2. There is exactly one missing number in the given sequence.
3. All the numbers present in the sequence are distinct.
4. It is the guarantee that the first and last elements of the sequence are not missing elements.
Follow Up
The overall run time complexity should be O(log(N)).
Input Format:
The first line of the input contains an integer ‘T’ denoting the number of test cases.

The first line of each test case contains a single positive integer ‘N’ denoting the number of the elements present in the sequence. 

The second line of each test case contains ‘N’ space-separated integers.
Output Format:
The only line of output of each test case should contain an integer denoting the missing element in the given sequence.

Print the output of each test case in a separate line.
Constraints:
1 <= T <= 50
3 <= N <= 10 ^ 4   
-10 ^ 9 <= Arr[i] <= 10 ^ 9 

Where ‘T’ is the number of test cases, ‘N’ is the size of the array and ‘Arr[i]’ is the size of the array elements.

Time Limit: 1 sec
Sample Input 1:
2
3
1 4 10
4
5 10 20 25 
Sample Output 1:
 7
 15
Explanation for sample input 1:
Test case 1:

The arithmetic sequence present in the first test case will have its first term as 4 and common difference as 3. So, the complete sequence will look like this ....- 1 4 7 10... Hence 7 is the missing element in the given sequence.

Test case 2:

The first term and common difference will be 5. The complete sequence will be ...5 10 15 20 25.... Hence 15 is the missing element from the given sequence.
Sample Input 2:
3
3
-1 0 2
5 
10 20 30 50 60
4
12 18 21 24
Sample Output 2:
1
40
15
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