class Solution {
    public String convert(String s, int numRows) {
        if (numRows == 1) {
            return s;
        }
        StringBuilder[] sbs = new StringBuilder[numRows];
        for (int i = 0; i < numRows; i++) {
            sbs[i] = new StringBuilder();
        }
        int index = 0;
        while (index < s.length()) {
            for (int i = 0; i < numRows && index < s.length(); i++) {
                sbs[i].append(s.charAt(index++));
            }
            for (int i = numRows - 2; i > 0 && index < s.length(); i--) {
                sbs[i].append(s.charAt(index++));
            }
        }
        for (int i = 1; i < numRows; i++) {
            sbs[0].append(sbs[i]);
        }
        return sbs[0].toString();
    }
}