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# Runtime: 96 ms (Top 61.23%) | Memory: 14 MB (Top 75.15%)
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class Solution:
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    def convert(self, s: str, numRows: int) -> str:
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        # safety check to not process single row
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        if numRows == 1:
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            return s
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        # safety check to not process strings shorter/equal than numRows
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        if len(s) <= numRows:
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            return s
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        # safety check to not process double rows
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        if numRows == 2:
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            # slice every other character
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            return s[0::2] + s[1::2]
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        # list that stores the lines
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        # add lines with initial letters
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        lines: list[str] = [letter for letter in s[:numRows]]
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        # positive direction goes down
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        # lines are created, so it's going up
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        direction: int = -1
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        # track the position at which the letter will be added
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        # position after bouncing off, after adding initial lines
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        line_index: int = numRows - 2
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        # edge indexes
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        # 0 can only be reached by going up
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        # numRows only by going down
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        edges: set[int] = {0, numRows}
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        for letter in s[numRows:]:
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            # add letter at tracked index position
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            lines[line_index] += letter
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            # prepare index before next loop iteration
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            line_index += direction
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            # reaching one of the edges
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            if line_index in edges:
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                # change direction
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                direction = -direction
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                # bounce off if bottom edge
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                if line_index == numRows:
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                    line_index += direction * 2
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        return "".join(lines)