foo.cc: In function 'void Merge(int*, int, int*, int)':
foo.cc:6:2: error: expected initializer before 'int'
  int i=0,j=0;
  ^~~
foo.cc:8:9: error: 'i' was not declared in this scope
  while( i<na && j<nb ) 
         ^
foo.cc:8:17: error: 'j' was not declared in this scope
  while( i<na && j<nb ) 
                 ^
foo.cc:11:4: error: 'g' was not declared in this scope
    g[k++]=a[i++],i++;
    ^
foo.cc:13:4: error: 'g' was not declared in this scope
    g[k++]=b[j++],j++; 
    ^
foo.cc:17:14: error: 'g' was not declared in this scope
        cout<<g[i]<<" ";
              ^
foo.cc:17:23: error: expected '}' at end of input
        cout<<g[i]<<" ";
                       ^

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