[LeetCode] 6. ZigZag Conversion

Problem : https://leetcode.com/problems/zigzag-conversion/

The solution is just simulating zigzag traversal steps. Be careful that when numRows <= 1, the code should return the original string.

Time Complexity = O(len(s))
Space Complexity = O(len(s))

from operator import add

class Solution(object):
    def convert(self, s, numRows):
        """
        :type s: str
        :type numRows: int
        :rtype: str
        """
        
        if numRows <= 1:
            return s
        
        memo = [""] * numRows
        
        i = 0
        row = 0
        direction = 1
        col = 0
        
        while i < len(s):
            memo[row] += s[i]
            
            row = row + direction
            if row == numRows:
                row = numRows - 2
                direction = - 1
            
            if row < 0:
                row = 1
                direction = 1
            
            i += 1
            
        return reduce(add, memo)