程序阅读（1）：字符串重排

using std::string;
string flip(string in)
{
    string out = "";
    for (unsigned i = 0; i < in.size(); ++i)
    {
        string buffer = "";
        buffer += in[i];
        for (unsigned j = out.size(); j > 0; --j)
        {
            buffer += out[j - 1];
        }
        out = buffer;
    }
    return out;
}

若 flip(s) == "pineapple"，则字符串 s 为（ ）。

{{ select(1) }}

• "apepnliep"
• "apnepilep"
• "apeplniep"
• "apenpliep"