leetcode--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 nRows) {
        int length = s.length();
        if(nRows == 0)
        	return null;
        if(length <= nRows  || nRows == 1)
        	return s;
        
        String[] zigzag = new String[nRows]; 
        for(int i = 0; i < nRows; ++i)
            zigzag[i] = "";
        int startIndex = 0;
        while(startIndex < length){
        	for(int j = 0 ; j < Math.min(length - startIndex, 2*nRows - 2); ++j){
        		if(j <  nRows)
        			zigzag[j] += s.substring(startIndex + j, startIndex + j + 1);
        		else
        			zigzag[2*nRows - 2 - j] +=  s.substring(startIndex + j, startIndex + j + 1);
        	}
        	startIndex += 2*nRows - 2;
        }
        
        //concatenate components
        String converted ="";        
        for(int i = 0; i < nRows; ++i)
        	converted += zigzag[i];        
        return converted;    
    }
}