Phase Shift

Description:

Input:

The first line of the input contains a single integer $$$t$$$ ($$$1 \le t \le 3 \cdot 10^4$$$) — the number of test cases. The description of test cases follows.

The first line of each test case contains one integer $$$n$$$ ($$$1 \le n \le 10^5$$$) — the length of the string $$$t$$$.

The next line contains the string $$$t$$$ of the length $$$n$$$, containing lowercase English letters.

It is guaranteed that the sum of $$$n$$$ over all test cases doesn't exceed $$$2 \cdot 10^5$$$.

Output:

For each test case, output a single line containing the lexicographically smallest string $$$s$$$ which could be a prototype of $$$t$$$.

Sample Input:

5
1
a
2
ba
10
codeforces
26
abcdefghijklmnopqrstuvwxyz
26
abcdefghijklmnopqrstuvwxzy

Sample Output:

b
ac
abcdebfadg
bcdefghijklmnopqrstuvwxyza
bcdefghijklmnopqrstuvwxyaz

Note:

In the first test case, we couldn't have the string "a", since the letter a would transit to itself. Lexicographically the second string "b" is suitable as an answer.

In the second test case, the string "aa" is not suitable, since a would transit to itself. "ab" is not suitable, since the circle would be closed with $$$2$$$ letters, but it must contain all $$$26$$$. The next string "ac" is suitable.

Below you can see the schemes for the first three test cases. The non-involved letters are skipped, they can be arbitrary placed in the gaps.