/in/foo.c: In function 'sort':
/in/foo.c:10:24: warning: iteration 20000 invokes undefined behavior [-Waggressive-loop-optimizations]
         for(j=1;j<=tong[i];j++)
                    ~~~~^~~
/in/foo.c:9:5: note: within this loop
     for(i=1;i<=20001;i++)
     ^~~
/in/foo.c: In function 'main':
/in/foo.c:29:16: warning: iteration 20000 invokes undefined behavior [-Waggressive-loop-optimizations]
         tong[i]=0;
         ~~~~~~~^~
/in/foo.c:28:5: note: within this loop
     for(i=1;i<=20001;i++)
     ^~~