foo.c: In function 'tree':
foo.c:39:11: warning: unused variable 'j' [-Wunused-variable]
     int i,j;
           ^
foo.c: In function 'main':
foo.c:72:5: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d",&n);
     ^~~~~~~~~~~~~~
foo.c:76:11: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
           scanf("%d",&a[i]);
           ^~~~~~~~~~~~~~~~~

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