B"You have an array a of n non-negative integers. Let's define f(a, x) = [a_1 bmod x, a_2 bmod x, ... , a_n bmod x] for some positive integer x . Find the biggest x , such that f(a, x) is a palindrome. Here, a bmod x is the remainder of the integer division of a by x . An array is a palindrome if it reads the same backward as forward. More formally, an array a of length n is a palindrome if for every i ( 1 <= q i <= q n ) a_i = a_{n - i + 1} . The first line contains a single integer t ( 1 <= q t <= q 10^5 ) -- the number of test cases. The first line of each test case contains a single integer n ( 1 <= q n <= q 10^5 ). The second line of each test case contains n integers a_i ( 0 <= q a_i <= q 10^9 ). It's guaranteed that the sum of all n does not exceed 10^5 . For each test case output the biggest x , such that f(a, x) is a palindrome. If x can be infinitely large, output 0 instead. In the first example, f(a, x = 1) = [0, 0] which is a palindrome. In the second example, f(a, x = 2) = [1, 0, 1, 0, 0, 1, 0, 1] which is a palindrome. It can be proven that in the first two examples, no larger x satisfies the condition. In the third example, f(a, x) = [0] for any x , so we can choose it infinitely large, so the answer is 0 . "...