foo.cc: In function 'int main()':
foo.cc:13:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     while (numb.size() < k) {//TO DO:找序列前k个
            ~~~~~~~~~~~~^~~
foo.cc:22:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < numb.size(); i++)//TO DO:int转string。
                     ~~^~~~~~~~~~~~~
foo.cc:35:15: warning: statement has no effect [-Wunused-value]
         for (i; i != n.end() ; i++)
               ^

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