ZigZagConversionPoor.java

public String convertSimulation(String s, int nRows) {
    if (s == null || s.length() == 0 || nRows <= 0) {
        return "";
    }

    if (nRows == 1) {
        return s;
    }

    int size = s.length();
    List<StringBuilder> buckets = new ArrayList();

    for (int i = 0; i < nRows; i++) {
        buckets.add(new StringBuilder());
    }

    int i = 0; 
    int index = 0;
    boolean fromTopToBottom = true;
    while (i < size) {
        char c = s.charAt(i);

        if (fromTopToBottom) {
            if (index == nRows - 1) {
                fromTopToBottom = false;
                buckets.get(index).append(c);
                index--;
            } else {
                buckets.get(index).append(c);
                index++;
            }
        } else {
            if (index == 0) {
                fromTopToBottom = true;
                buckets.get(index).append(c);
                index++;
            } else {
                buckets.get(index).append(c);
                index--;
            }
        }
        i++;
    }

    String res = "";
    for (StringBuilder sb : buckets) {
        res += sb.toString();
    }

    return res;
}