foo.cc: In function 'int main()':
foo.cc:123:6: warning: array subscript is above array bounds [-Warray-bounds]
  a[21]=1;
  ~~~~^
foo.cc:124:6: warning: array subscript is above array bounds [-Warray-bounds]
  b[21]=2;
  ~~~~^
foo.cc:125:6: warning: array subscript is above array bounds [-Warray-bounds]
  c[21]=3;
  ~~~~^

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