Leetcode 6 ZigZag Conversion

The string "PAYPALISHIRING" is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility)

P   A   H   N
A P L S I I G
Y   I   R

And then read line by line: "PAHNAPLSIIGYIR"

Write the code that will take a string and make this conversion given a number of rows:

string convert(string s, int numRows);

Example 1:

Input: s = "PAYPALISHIRING", numRows = 3
Output: "PAHNAPLSIIGYIR"

Example 2:

Input: s = "PAYPALISHIRING", numRows = 4
Output: "PINALSIGYAHRPI"
Explanation:

P     I    N
A   L S  I G
Y A   H R
P     I

class Solution {
    public String convert(String s, int numRows) {
        if(s == null || s.length() == 0 || numRows <= 1){
            return s;
        }
        int len = s.length();
        int circle = 2 * numRows - 2;
        StringBuilder[] sc = new StringBuilder[numRows];
        for (int i = 0; i < sc.length; i++)
            sc[i] = new StringBuilder();
        for(int i = 0 ; i < len; i++){
            int num = i % circle;
            if(num < numRows){
                char c = s.charAt(i);
                sc[num].append(c);
            }else{
                sc[circle - num].append(s.charAt(i));
            }
        }
        StringBuilder ss = new StringBuilder();
        for(int i = 0 ; i < numRows ; i++){
            ss.append(sc[i]);
        }
        return ss.toString();
    }
}

時間複雜度為O（n），思路是借鑑的牛客上面的，很奇妙，自己手動擼了一遍基本掌握-