How to write to a file in Kotlin

Java Programming interview coding question.

Question: 

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 1:

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

Java solution

public class MainClass {
    public static void main(String[] args) {
        String convertedString = convert("PAYPALISHIRING", 4);        System.out.println(convertedString);        /**P A H N         * APLSIIG         * Y I R         *=PAHNAPLSIIGYIR         */    }

    public static String convert(String s, int numRows) {
        if(numRows == 1) return s;
        List<StringBuilder> rows = new ArrayList<>();        for(int i = 0; i < Math.min(numRows, s.length()); i++){
            rows.add(new StringBuilder());        }

        int curRow = 0;        boolean goingDown = false;
        for(char c: s.toCharArray()){
            rows.get(curRow).append(c);            if(curRow == 0 || curRow == numRows - 1){
                goingDown = !goingDown;            }
            curRow += goingDown ? 1 : -1;        }

        StringBuilder ret = new StringBuilder();        for(StringBuilder row: rows){
            ret.append(row);        }
        return  ret.toString();
    }
}