foo.cc: In function 'int main()':
foo.cc:9:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i = 0;i < k.length();i++){
                   ~~^~~~~~~~~~~~
foo.cc:5:9: warning: variable 'n' set but not used [-Wunused-but-set-variable]
     int n;
         ^