ZigZag Conversion

Problem:

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".

Analysis:
For the first and the last rows, the period between tow items is 2nRow-2(Step). For other rows, the period changes after every step. The first period is 2(nRows-1-rowIndex)(Step1), the second period is Step-Step1. 

Solution:

1:  public class Solution {  
2:    public String convert(String s, int nRows) {  
3:      if(nRows==1)  
4:       return s;  
5:      StringBuilder sb=new StringBuilder();   
6:      int step=2*nRows-2;//the period on the first and the last rows  
7:      for(int i=0;i<nRows;i++)  
8:      {  
9:        if(i==0 || i==nRows-1)//first row or last row  
10:        {  
11:         for(int j=i;j<s.length();j+=step)  
12:         {  
13:           sb.append(s.charAt(j));  
14:         }  
15:        }  
16:        else  
17:        {  
18:          int j=i;  
19:          int substep=2*(nRows-1-i);  
20:          while(j<s.length())  
21:          {  
22:            sb.append(s.charAt(j));  
23:            j+=substep;  
24:            substep=step-substep;    
25:          }  
26:        }  
27:      }  
28:      return sb.toString();  
29:    }  
30:  }