This one took me about 15 minutes to come up with a solution. I then worked through it with this naive solution.

---

Solution

class Solution:
    def convert(self, s: str, numRows: int) -> str:
        grid = [['' for column in range(len(s))] for row in range(numRows)]
        x, y = 0, 0
        direction = 1
        for char in s:
            grid[y][x] = char
            if y == numRows - 1:
                direction = -1
                x += 1
            elif direction == -1 and y > 0:
                x += 1
            elif direction == -1 and y == 0:
                direction = 1
            y += direction if numRows > 1 else 0
        result = ""
        for row in grid:
            for char in row:
                if char:
                    result += char
        return result

O(n) = n*m or n^2

---

Reflection

Looking back, I realized I didn’t need to include the empty spaces as if it were illustrated. This allows me to simplify my code by using the append() method for lists. Here is my improved version:

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

        grid = [[] for _ in range(numRows)]
        y = 0 
        direction = 1
        for char in s:
            grid[y].append(char)
            if y == numRows - 1:
                direction = -1
            elif y == 0:
                direction = 1
            y += direction
        
        for i in range(len(grid)):
            grid[i] = ''.join(grid[i])
        result = ''.join(grid)
        return result