思路1 利用字符串list进行转换

我们创建一个字符串数组来存放结果，这个数组的长度就是numofRows。遍历原字符串，不断的将遍历到的字符加载每一行的后面，遇到boundary则改变step的方向。这里的boundary就是0和numofRows-1。

class Solution {
public:
    string convert(string s, int numRows) {
        if (numRows == 1 || numRows >= s.size()) return s;
        vector<string> res(numRows, "");
        int step;
        int cur_row = 0;

        for (int i = 0; s[i]; i++) {
            res[cur_row] += s[i];
            if (cur_row == numRows - 1) {
                step = -1;
            } else if (cur_row == 0) {
                step = 1;
            }
            cur_row += step;
        }
        string res_str = "";
        for (int i = 0; i < res.size(); i++) res_str += res[i];
        return res_str;
    }
};

在python中我们直接用''.join(list_name)的形式把list转换为字符串

class Solution:
    def convert(self, s: str, numRows: int) -> str:
        if numRows >= len(s) or numRows == 1:
            return s
        step = 1
        index = 0
        res = [''] * numRows
        for i, v in enumerate(s):
            # cur_line is not index
            res[index] += v
            if index == numRows-1:
                step = -1
            elif index == 0:
                step = 1
            index += step
        return ''.join(res)