LeetCode ZigZag Conversion

Implimentation:
index+unitsize - 2*row

public String convert(String s, int nRows) {
        //Boundary condition
        if (s==null || s.length()==0 || nRows <=0) return "";
        if (nRows==1) return s;
        //Traverse the whole string, chopped the string as different unit
        //The unit size should be 2*nRows - 2
        //In each unit, the first column is easy to handle.
        //The second column need to use index + Unitsize - 2 * row
        int unitSize = 2*nRows - 2;
        StringBuilder result = new StringBuilder();
        for (int i=0; i<nRows; i++) {
            for (int j = i; j < s.length(); j+=unitSize) {
                result.append(s.charAt(j));
                if (i!=0 && i!=nRows-1 && j+unitSize-2*i<s.length()) {
                    result.append(s.charAt(j+unitSize-2*i));
                }
            }
        }
        return result.toString();
    }