6ZigZag 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 s, int numRows);
Example 1:

Input: s = "PAYPALISHIRING", numRows = 3
Output: "PAHNAPLSIIGYIR"
Example 2:

Input: s = "PAYPALISHIRING", numRows = 4
Output: "PINALSIGYAHRPI"
Explanation:

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

rule . 2 * numRows - 2

A

B E

C D

D

int index = i % (2 * numRows - 2);
index = (index < numRows) ? index :(2 * numRows - 2 - index);
sb[index].append(s.charAt(i));

class Solution {
    public String convert(String s, int numRows) {
        if (numRows <= 1) {
            return s;
        }
        StringBuilder[] sb = new StringBuilder[numRows];
        for (int i = 0; i < sb.length; i++) {
            sb[i] = new StringBuilder();
        }
        for (int i = 0; i < s.length(); i++) {
            int index = i % (2 * numRows - 2);
            index = (index < numRows) ? index :(2 * numRows - 2 - index);
            sb[index].append(s.charAt(i));
        }
        String ans = "";
        for (int i = 0; i < sb.length; i++) {
            ans += sb[i].toString();
        }
        return ans;
    }
}