Zig-Zag String Problem Statement

Given a string 'STR' of length 'N' and an integer 'M' representing the number of rows in the zig-zag pattern, your task is to return the string formed by concatenating all 'M' rows when 'STR' is arranged in a zig-zag pattern.

Input:

The first line contains an integer 'T' representing the number of test cases.
Each test case begins with two integers 'N' and 'M', representing the length of the string 'STR' and the number of rows, respectively.
The next line contains the string 'STR'.

Output:

For each test case, return the concatenated string from the zig-zag pattern of 'STR' over 'M' rows.

Example:

Input:

N = 12, M = 3, STR = 'CODINGNINJAS'

Output:

'CNNOIGIJSDNA'

Explanation:

In a zig-zag pattern with 3 rows, row 1 is 'CNN', row 2 is 'OIGIJS', and row 3 is 'DNA'. The concatenation is 'CNNOIGIJSDNA'.

Constraints:

• 1 ≤ T ≤ 102
• 1 ≤ N ≤ 103
• 1 ≤ M ≤ N
• The string 'STR' contains only uppercase letters ('A-Z').