leetcode 6 ZigZag

string convert(string s, int nRows) {

    if (nRows <= 1)
        return s;

    const int len = (int)s.length();
    string *str = new string[nRows];//设置nrows个字符串

    int row = 0, step = 1;
    for (int i = 0; i < len; ++i)
    {
        str[row].push_back(s[i]);

        if (row == 0)
            step = 1;
        else if (row == nRows - 1)
            step = -1;

        row += step;
    }

    s.clear();
    for (int j = 0; j < nRows; ++j)
    {
        s.append(str[j]);
    }

    delete[] str;
    return s;
}