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ZigZag Conversion

6. 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".

public class Solution {  
    public String convert(String s, int numRows) {
        if (s == null || s.length() == 0 || numRows <= 0) {
            return "";
        }

        if (numRows == 1) {
            return s;
        }

        StringBuilder sb = new StringBuilder();
        int size = 2 * numRows - 2;

        for (int i = 0; i < numRows; i++) {
            for (int j = i; j < s.length(); j += size) {
                sb.append(s.charAt(j));

                if (i != 0 && i != numRows - 1) {
                    int temp = j + size - 2 * i;

                    if (temp < s.length()) {
                        sb.append(s.charAt(temp));
                    }
                }
            }
        }

        return sb.toString();
    }
}

Hope this helps,
Michael