Zigzag Conversion

Zigzag Conversion

What is this problem about?

The Zigzag Conversion coding problem is a classic string manipulation task. Given a string and a specific number of rows, you are asked to "write" the string in a zigzag pattern across those rows and then read it line by line. For example, in a 3-row zigzag, the characters go down the first column, then diagonally up to the right, then down again. The goal is to return the resulting string after reading the rows from top to bottom.

Why is this asked in interviews?

This Zigzag Conversion interview question is legendary and has been asked at almost every major tech company, including Apple, Microsoft, and Amazon. It is a pure test of your "Implementation" skills. It doesn't require complex algorithms or data structures; instead, it requires you to carefully manage indices and handle edge cases (like when the number of rows is 1). It shows an interviewer that you can take a wordy, visual description and translate it into clean, bug-free code.

Algorithmic pattern used

The primary algorithmic pattern is Simulation. You can use an array of strings (or string builders), where each index represents a row. You iterate through the input string, keeping track of the "current row" and the "direction" (moving down or moving up). When you hit the top or bottom row, you reverse the direction. After placing all characters into their respective rows, you simply concatenate the rows to get the final answer.

Example explanation

Take the string "PAYPALISHIRING" and numRows = 3. The pattern looks like this:

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

Row 1: PAHN Row 2: APLSIIG Row 3: YIR Reading row by row, you get: "PAHNAPLSIIGYIR". The Zigzag Conversion coding problem asks you to generate this transformed string for any given input and number of rows.

Common mistakes candidates make

A very common mistake in the Zigzag Conversion interview question is failing to handle the numRows = 1 case, which often leads to an infinite loop or index out of bounds because the "direction" never actually changes. Another error is miscalculating the period of the zigzag (which is 2 * numRows - 2) if trying to solve it mathematically without simulation. Some candidates also struggle with the diagonal placement, placing characters in the wrong rows.

Interview preparation tip

When solving the Zigzag Conversion interview pattern, focus on the "State Machine" approach. Maintain a variable for the currentRow and a boolean for goingDown. This makes the logic very easy to follow and debug. Also, remember to handle cases where the string length is less than the number of rows. Before writing code, trace a small example on paper to ensure your row-switching logic is sound.