6. Z 字形变换

class Solution {
    public String convert(String s, int n) {
        if (n == 1) {
            return s;
        }
        int len = s.length();
        StringBuilder res = new StringBuilder();
        for (int i = 0; i < n; i++) {
            if (i == 0 || i == n - 1) {
                for (int j = i; j < len; j += 2 * n - 2) {
                    res.append(s.charAt(j));
                }
            } else {
                for (int j = i, k = 2 * n - 2 - i; j < len|| k < len; j += 2 * n - 2, k += 2 * n - 2) {
                    if (j < len) {
                        res.append(s.charAt(j));
                    }
                    if (k < len) {
                        res.append(s.charAt(k));
                    }
                }
            }
        }
        return res.toString();
    }
}

• 时间复杂度O(n)
• 空间复杂度O(n)