ZigZagConversionGood.java

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

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

    int size = s.length();
    // directly allocate a string builder array
    StringBuilder[] buckets = new StringBuilder[nRows];
    for (int i = 0; i < nRows; i++) {
        buckets[i] = new StringBuilder();
    }


    int i = 0;
    int index = 0;
    boolean fromTopToBottom = true;
    while (i < size) {
        char c = s.charAt(i);
        buckets[index].append(c);
        index += fromTopToBottom ? 1 : -1;

        if (index == nRows - 1 || index == 0) {
            fromTopToBottom = !fromTopToBottom;
        }

        i++;
    }

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

    return res;
}