原题链接: https://oj.leetcode.com/problems/zigzag-conversion/

public String convert(String s, int nRows) {
    if (s == null || nRows <= 1) return s;

    StringBuilder sb = new StringBuilder();
    int length = s.length();
    int span = 2 * nRows - 2;

    for (int i = 0; i < nRows; i++) {
        for (int j = i; j < length; j += span) {
            sb.append(s.charAt(j));

            if (i != 0 && i != nRows - 1 && j + span - 2 * i < length) {
                sb.append(s.charAt(j + span - 2 * i));
            }
        }
    }

    return sb.toString();
}