foo.cc: In function 'int trap(int*, int)':
foo.cc:21:5: error: 'volume' was not declared in this scope
     volume += right - left + 1; 
     ^~~~~~
foo.cc: In function 'int main()':
foo.cc:28:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&T);
  ~~~~~^~~~~~~~~
foo.cc:31:8: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d",&N);
   ~~~~~^~~~~~~~~
foo.cc:33:8: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d",&a[j]);
   ~~~~~^~~~~~~~~~~~

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