foo.cc: In function 'int main()':
foo.cc:20:5: error: 'ptr' was not declared in this scope
     ptr = set_union(a, a + N, b, b + M, unionset);
     ^~~
foo.cc:20:5: note: suggested alternative: 'putw'
     ptr = set_union(a, a + N, b, b + M, unionset);
     ^~~
     putw

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