foo.cc: In function 'int f(int)':
foo.cc:15:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int j=0;j<a[x].size();j++){
              ~^~~~~~~~~~~~
foo.cc:19:15: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int j=0;j<a[x].size();j++){
              ~^~~~~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:27:6: error: 'i' was not declared in this scope
  for(i=1;i<=k;i++){
      ^
foo.cc:45:6: error: 'i' was not declared in this scope
  for(i=1;i<=k;i++){
      ^
foo.cc:26:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d%d",&k,&n);
  ~~~~~^~~~~~~~~~~~~~
foo.cc:31:9: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
    scanf("%d%d",&x,&y);
    ~~~~~^~~~~~~~~~~~~~

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