Zig-Zag String Problem Statement

Given a string STR of size N and an integer M representing the number of rows in the zig-zag pattern, return the string formed by concatenating all rows when the string STR is written in a row-wise zig-zag pattern.

Example:

Input:

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

Output:

'CNNOIGIJSDNA'

Explanation:

When arranged in a zig-zag pattern with 3 rows, the rows contain the following letters: Row 1: 'CNN', Row 2: 'OIGIJS', Row 3: 'DNA'. Concatenating these rows gives the result: 'CNNOIGIJSDNA'.

Constraints:

• 1 <= T <= 10^2
• 1 <= N <= 10^3
• 1 <= M <= N
• STR contains only uppercase letters ('A-Z').
• Time Limit: 1 second