6. Z 字形变换

string convert(string s, int numRows) {
    if (numRows < 2)
        return s;
    vector<string> v(numRows);
    int index = 0;
    //01 21 01 2
    int flag = -1;
    for (int i = 0; i < s.size(); i++) {
        v[index] += s[i];
        if (index == 0 || index == numRows - 1)
            flag = -flag;
        index += flag;
    }

    string str;
    for (int i = 0; i < numRows; i++)
        str += v[i];
    return str;
}