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:

convert("PAYPALISHIRING", 3) should return "PAHNAPLSIIGYIR".

nRow = 2:

0 2 4 6 8 10 12
1 3 5 7 9 11 13

nRow = 3: 

0     4      8     12 
1  3  5   7  9  11
2     6     10

nRow = 4:
0      6       12
1   5  7    11 13
2 4    8  10
3      9

from the examples we can see the formula: from first non-zigzag column to next non-zigzag column, indexDiff = 2*nRows-2;

if not the first row and not the last row, the index difference between two char are columnIndexDiff = j+indexDiff-2*currRow

j is non zigzag column index

public String convert(String s, int nRows) {
        if (s==null||s.isEmpty()||nRows<1) {
            return "";
        }
        if (nRows==1) {
            return s;
        }

        StringBuilder results = new StringBuilder();
        int size = 2*nRows-2;

        for(int i=0;i<nRows;i++)  {  
            for(int j=i;j<s.length();j+=size) {  
                results.append(s.charAt(j));  
                //if not first and last row, we need to append the char in between.
                int nextZig = j+size-2*i; //i is current row
                if(i!=0 && i!=nRows-1 && nextZig<s.length()) {
                    results.append(s.charAt(nextZig));  
                }                    
            }                  
        }  
        return results.toString();
    }