Directi Internship test2

You are given a rectangular grid with 2 rows and N columns. The top row is labeled 1 and the bottom row is labeled 2. The columns are labeled from 1 to N in increasing order. Each cell in the grid contains a single character.
Consider a hamiltonian walk in this grid. Meaning, pick a starting cell, say (i,j), and consider a path that starts from (i,j) and goes through every cell in the grid exactly once. Note that you can only walk to adjacent cells, or cells that you share a common edge with. There may be several such paths. Let us concatenate the characters in the order in which the cells are visited during a walk. The string formed can be called the string for the walk.
Among all the possible walks, and their respective strings, find out the lexicographically smallest string. We know that the length of the strings are all the same - to be precise, 2N. Thus, the lexicographically smallest string is simply the alphabetically smallest string if you compare the characters from left to right.
Input

The first line of input contains a number T, the number of test cases. Then follow T test cases. Each test case contains 3 lines. The first line contains the number N, the number of columns in the grid. It is well known of course that the grid contains 2 rows. The next two lines contain the description of the grid in the form of two strings; the string of N characters in row 1 from left to right and the string of N characters in row 2 from left to right, respectively. Each character will be a lowercase engish letter.
Output

Output a single line for each test case. The line must contain a string with 2N characters. This string should be the lexicographically smallest string for some hamiltonian walk in the grid.
Constraints

1 = T = 100
1 = N = 10

Sample Input

2
3
abc
def
10
ababaaabab
bababababa

Sample Output

abcfed
aaababababababababab

Explanation

In the first test the possible strings are { abcfed, adebcf, adefcb, badefc, bcfeda, cbadef, cfedab, cfebad, dabcfe, dabefc, defcba, edabcf, efcbad, fedabc, fcbade, fcbeda }. The smallest string is abcfed.