ZigZag Conversion

public class Solution {
    public String convert(String s, int numRows) {
        // illegal parameters
        if(s == null || s.length() == 0 || numRows <= 0) {
            return s;
        }
        
        if(s.length() <= numRows || numRows == 1) {
            return s;
        }
        
        
        // get column numbers of array
        int numCols = 0;
        int len = s.length();
        while(len > 0) {
            len -= numRows * 2 - 2;
            numCols += numRows - 1;
        }
        
        StringBuilder res = new StringBuilder();
        Character[][] array = new Character[numRows][numCols];
        boolean isDown = true;
        int rowIndex = 0;
        int colIndex = 0;
        
        for(int i = 0; i < s.length(); i++) {
            Character c = s.charAt(i);
            array[rowIndex][colIndex] = c;
            if(isDown) {
                // going down direction
                rowIndex++;
                if(rowIndex == numRows) {
                    // time to change to lean direction
                    isDown = false;
                    rowIndex -= 2;
                    colIndex++;
                }
            } else {
                // going lean direction
                if(rowIndex == 0) {
                    // time to change to down direction
                    isDown = true;
                    rowIndex++; 
                } else {
                    rowIndex--;
                    colIndex++;
                }
            }
        }
        
        for(int i = 0; i < numRows; i++) {
            for(int j = 0; j < numCols; j++) {
                Character c = array[i][j];
                if(c == null) {
                    continue;
                } else {
                    res.append(c);
                }
            }
        }
        
        return res.toString();
    }
}