6. Zig Zag Conversion

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

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 text, int nRows);

convert("PAYPALISHIRING", 3) should return "PAHNAPLSIIGYIR".
Ref: Zig Zag conversion solution

/*n=numRows
Δ=2n-2    1                           2n-1                         4n-3
Δ=        2                     2n-2  2n                    4n-4   4n-2
Δ=        3               2n-3        2n+1              4n-5       .
Δ=        .           .               .               .            .
Δ=        .       n+2                 .           3n               .
Δ=        n-1 n+1                     3n-3    3n-1                 5n-5
Δ=2n-2    n                           3n-2                         5n-4
*/

public class Solution {
    public String convert(String s, int numRows) {
        if (numRows == 1) {
            return s;
        }
        
        int step = 2 * numRows - 2;
        
        StringBuilder sb = new StringBuilder();
        
        for (int i = 0; i < numRows; i++) {
            if (i == 0 || i == numRows - 1) { // first and last rows
                for (int j = i; j < s.length(); j = j + step) {
                     sb.append(s.charAt(j));
                }
            } else {
                // handle  middle rows
                int j = i;
                boolean flag = true;
                int step1 = 2 * (numRows - 1 - i);
                int step2 = step - step1;
                
                while (j < s.length()) {
                    sb.append(s.charAt(j));
                    if (flag) {
                        j = j + step1;
                    } else {
                        j = j + step2;
                    }
                    flag = !flag;
                }
            }
        }
        return sb.toString();
    }
}