[LeetCode] Zigzag Conversion

Problem: The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)

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

And then read line by line: "PAHNAPLSIIGYIR"

Write the code that will take a string and make this conversion given a number of rows:

string convert(string s, int numRows);

Example:

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

Approach:

1. Create an array of n strings say result
2. Initialize increase to 1 and row as 0. The increase indicates whether we need to move up or down in rows. 
3. Traverse the input string, do following for every character.

•  Append current character to string of current row. 
•  row = row + increase.
• If row number is n-1 or 0, multiply increase with -1.

Implementation in C#:

    public string Convert(string s, int numRows)

    {

        int length = s?.Length ?? 0;

        if (length <= 1 || numRows == 1)

        {

            return s;

        }

        StringBuilder[] rows = new StringBuilder[numRows];

        for (int i = 0; i < numRows; ++i)

        {

            rows[i] = new StringBuilder();

        }

        int r = 0, increase = 1;

        for (int i = 0; i < length; ++i)

        {

            rows[r].Append(s[i]);

            r += increase;

            if (r % (numRows - 1) == 0)

            {

                increase *= -1;

            }

        }

        StringBuilder result = new StringBuilder();

        foreach (var row in rows)

        {

            result.Append(row);

        }

        return result.ToString();

    }

    

Complexity: O(n)