Problem URL: Zigzag conversion

We will create an array with empty string for each row. Then we also have a direction value, which will be down at the beginning. Then we start from position 0, add items in each row until we reach the last row, then change our direction to up, and add items to each row. We will continue this until we reach end of the string, then we combine each row to a string and return that value.

class Solution:
    def convert(self, s: str, numRows: int) -> str:
        if numRows == 1:
            return s

        rows = ["" for i in range(numRows)]
        pos = 0
        down = True

        for ch in s:
            rows[pos] += ch
            pos += 1 if down else -1
            if pos == numRows-1 or pos == 0:
                down = not down

        return ''.join(rows)

Time Complexity: O(n)
Space Complexity: O(n)