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".

public class Solution {
    public String convert(String s, int numRows) {
        int pStr = 0;
        String ret = "";
        String[] graph = new String[numRows];
        for(int i = 0; i < numRows; i++){
            graph[i] = "";
        }
        
        while(pStr < s.length()){
            for(int i = 0; i < numRows; i++){
                graph[i] += s.charAt(pStr++); 
                if(pStr >= s.length()) break;
            }
            if(pStr >= s.length()) break;
            
            for(int i = numRows-2; i > 0; i--){ //even column, 2 element shorter than the odd column, fill from bottom to top
                graph[i] += s.charAt(pStr++); 
                if(pStr >= s.length()) break;
            }
        }
        
        for(int i = 0; i < numRows; i++){
            ret += graph[i];
        }
        return ret;
    }
}