foo.cc: In function 'int countChar(std::string, char)':
foo.cc:56:15: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   56 |  for(int i=0;i<s.size();i++)
      |              ~^~~~~~~~~
foo.cc: In function 'void maxChar(std::string)':
foo.cc:79:11: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   79 |  for(i=0;i<s.length();i++)
      |          ~^~~~~~~~~~~
foo.cc:81:9: warning: array subscript has type 'char' [-Wchar-subscripts]
   81 |   m[s[i]]++;
      |         ^
foo.cc: In function 'int longestSubstr(std::string)':
foo.cc:139:6: warning: unused variable 't' [-Wunused-variable]
  139 |  int t,i,n,len,rk;
      |      ^
foo.cc: In function 'int longestSubstr(std::string, int)':
foo.cc:163:8: warning: 'left' may be used uninitialized in this function [-Wmaybe-uninitialized]
  163 |  int i,left,right,n;
      |        ^~~~

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