class Solution {
public String convert(String s, int nRows) {
    char[] c = s.toCharArray();
    int len = c.length;
    StringBuffer[] sb = new StringBuffer[nRows];
    for (int i = 0; i < sb.length; i++) sb[i] = new StringBuffer();

    int i = 0;
    while (i < len) {
        for (int idx = 0; idx < nRows && i < len; idx++) // vertically down
            sb[idx].append(c[i++]);
        for (int idx = nRows-2; idx >= 1 && i < len; idx--) // obliquely up
            sb[idx].append(c[i++]);
    }
    for (int idx = 1; idx < sb.length; idx++)
        sb[0].append(sb[idx]);
    return sb[0].toString();
}
}
