ex11_31 for the same elem

[Layty]

If the multimap has same elem like that

    std::multimap<string, string> authors{
        { "alan", "DMA" },
        { "pezy", "LeetCode" },
        { "alan", "CLRS" },
        { "wang", "FTP" },
        { "pezy", "CP5" },
        { "pezy", "CP5" },
        { "wang", "CPP-Concurrency" }
    };

It will just erase the first { "pezy", "CP5" },

[pezy]

Maybe the ‘break’ should be removed.