Leetcode 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"

.

Analysis: you have to pay attention to the order that the String traverse: How to get
the index of the String:

public class Solution {
public String convert(String s, int numRows) {
int length = s.length();
StringBuffer result = new StringBuffer();
char[] sToArray = s.toCharArray();

if(numRows == 1)
return s;

//This is to append the first row.
int next = 0;
while(next < length){
result.append(sToArray[next]);
next = next + 2*(numRows - 1);
}

for(int i = 1; i < numRows - 1; i++){
boolean zig = true;
int begin = i;
int nextEle = i;
while(nextEle < length){
//This is to return the result;
result.append(sToArray[nextEle]);
//first start with from top to bottom zig order
if(zig){
nextEle = nextEle + 2*(numRows - i - 1);
zig = false;
}
else{
nextEle = nextEle + i *2;
zig = true;
}
}
}

//This is to append the elements in the last row.
next = numRows - 1;
while(next < length){
result.append(sToArray[next]);
next = next + 2*(numRows - 1);
}
return result.toString();

}
}