feat: add zigzag conversion algorithm

strings/zigzag_conversion.py

@@ -0,0 +1,49 @@
+from __future__ import annotations
+
+
+def zigzag_convert(s: str, num_rows: int) -> str:
+    """
+    Convert a string to zigzag pattern and read it row by row.
+
+    The string is written in a zigzag pattern on num_rows rows:
+    - Characters at index 0, num_rows, 2*num_rows, ... go to row 0
+    - Characters at index 1, num_rows+1, 2*num_rows+1, ... go to row 1
+    - And so on until row num_rows-1
+    - Then the direction reverses (going up)
+
+    Args:
+        s: The input string to convert
+        num_rows: Number of rows in the zigzag pattern
+
+    Returns:
+        The string read row by row from top to bottom
+
+    >>> zigzag_convert("PAYPALISHIRING", 3)
+    'PAHNAPLSIIGYIR'
+    >>> zigzag_convert("PAYPALISHIRING", 4)
+    'PINALSIGYAHRPI'
+    >>> zigzag_convert("A", 1)
+    'A'
+    >>> zigzag_convert("AB", 1)
+    'BA'
+    """
+    if num_rows == 1 or num_rows >= len(s):
+        return s
+
+    rows: list[list[str]] = [[] for _ in range(num_rows)]
+    current_row = 0
+    going_down = False
+
+    for char in s:
+        rows[current_row].append(char)
+        if current_row == 0 or current_row == num_rows - 1:
+            going_down = not going_down
+        current_row += 1 if going_down else -1
+
+    return "".join("".join(row) for row in rows)
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()