foo.cc: In function 'int main()':
foo.cc:13:12: warning: unused variable 'j' [-Wunused-variable]
     int i, j;
            ^
foo.cc:18:14: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
         scanf("%c", &ch);
         ~~~~~^~~~~~~~~~~
foo.cc:35:14: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
         scanf("%s", str[idx[i]]);
         ~~~~~^~~~~~~~~~~~~~~~~~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。