Mahdi Shamlou | Solving LeetCode #6: Zigzag Conversion — My Row-by-Row Simulation Fun Way

#python #leetcode #programming #algorithms

Hey everyone! Mahdi Shamlou here — rolling on with my LeetCode classic problems series 🚀

After [#5 Longest Palindromic Substring (See the list that i solved)], today we’re tackling Problem #6 — Zigzag Conversion (also called ZigZag Conversion) — a cool medium-level problem that feels like drawing a zigzag pattern with letters!

Problem Statement

The string s is written in a zigzag pattern on a given number of rows like this (imagine fixed font):

For s = “PAYPALISHIRING”, numRows = 3:

P   A   H   N
A P L S I I G Y
I   R

Read line by line: “PAHNAPLSIIGYIR”

Write a function to convert the string this way given numRows.

Examples:

Input: s = "PAYPALISHIRING", numRows = 3
Output: "PAHNAPLSIIGYIR"

Input: s = "PAYPALISHIRING", numRows = 4
Output: "PINALSIGYAHRPI"
Explanation:
P     I    N
A   L S  I G Y
A   H R
P     I

Input: s = "AB", numRows = 2
Output: "AB"

My solution :

The simulation way with direction flip (this felt natural!) I imagined walking down the rows, then up, then down again — like a real zigzag. So I used a list of strings (one per row), a counter for current row, and a direction flag that flips at top/bottom.

class Solution:
    def convert(self, s: str, numRows: int) -> str:
        result = []
        numRows_counter = 0
        numRows_counter_way = 1

        for i in s:
            result.append("")  # wait, this was a bug — should init outside!
            result[numRows_counter] = str(result[numRows_counter]) + str(i)

            if numRows_counter + 1 == numRows:
                numRows_counter_way = -1
            if numRows_counter - 1 < 0:
                numRows_counter_way = +1

            if numRows_counter_way == 1:
                numRows_counter = numRows_counter + 1
            else:
                numRows_counter = numRows_counter - 1

        final_res = ""
        for i in result:
            final_res = final_res + i

        print(result)
        return final_res

Note: In my first run I had result = [] and appended empty string each time — that’s not correct! (It would create too many empty slots.) The proper way is to initialize result = [“”] * numRows outside the loop. But hey, this direction-flip idea still clicked for me after fixing that 😅

My repo (all solutions are here):

GitHub — mahdi0shamlou/LeetCode: Solve LeetCode Problems

What about you? Did you simulate the zigzag with rows like me, or did you go for the math formula way (cycle length = 2*numRows-2)? Any funny bugs you hit on this one?

Share in the comments — I read everything! 🚀

And honestly… after seeing “PINALSIGYAHRPI” come out correctly I just sat there grinning like an idiot — that zigzag finally made sense in code! 😂 Here’s me right after it worked: