Naive solution to Satvim's puzzle

SatvimPuzzle.java

package test_crawler.test;

import java.util.ArrayList;
import java.util.List;


/**
 * 1. Create a 2D array of characters 
 * 2. Gather the lower edge indexes first e.g. in a 3 x 3 array gather all y positions alone
 * y x x
 * y x x
 * y y y
 * 3. For each character in the input start with the co-ordinates gathered earlier and fill the 2D array with
 * characters until it reaches the other edge i.e. diagonally navigate the array
 * @author raam
 *
 */
public class SatvimPuzzle {

	private static final Index INVALID = new Index(-1, -1);
	
	static class Index {
		
		final int i;
		final int j;
		
		public Index(int x, int y){
			i = x;
			j = y;
		}		
	}
	
	private static List<Index> getLowerEdgeIndices(int numRows,int numCols){
		List<Index> indexes = new ArrayList<SatvimPuzzle.Index>();
		for(int i=0;i<numRows;i++)
			indexes.add(new Index(i,0));
		for(int i=1;i<numCols;i++)
			indexes.add(new Index(numRows-1,i));
		return indexes;
	}
	
	/**
	 * Returns INVALID if we have reached the other edge
	 * @param current
	 * @param result
	 * @return
	 */
	private static Index getNextIndex(Index current, char[][] result){
		if(current.i == 0 || current.j >= result[0].length-1)
			return INVALID;
		else
			return new Index(current.i-1, current.j+1); //return top right index
	}
	
	private static String buildStr(char[][] ret){
		StringBuilder sb = new StringBuilder();
		for(char[] arr: ret)
			sb.append(new String(arr).trim());
		return sb.toString();
	}
	
	public static String convert(String str,int numRows){
		int numCols  = (int)Math.ceil((double)str.length() / numRows);
		char[][] ret = new char[numRows][numCols];
		List<Index> indexes = getLowerEdgeIndices(numRows, numCols);
		int idxCounter = 1;
		Index temp = indexes.get(0);
		for(char c: str.toCharArray()){
			ret[temp.i][temp.j] = c;
			temp = getNextIndex(temp, ret);
			//if we reach the top edge proceed with next lower edge index
			if(temp == INVALID && idxCounter < indexes.size()) 
				temp = indexes.get(idxCounter++);
		}
		return buildStr(ret);
	}
	
	public static void main(String[] args) {
		System.out.println(convert("PAYPALISTHEFASTERSAFERWAYTOSENDMONEY", 4));
	}
	
}