/in/foo.cc: In function 'int check(int)':
/in/foo.cc:28:5: warning: operation on 'k' may be undefined [-Wsequence-point]
   b[++k]=a[k].y;
     ^~~
/in/foo.cc:29:30: warning: operation on 'k' may be undefined [-Wsequence-point]
   while (a[k].x==a[k+1].x) b[++k]=a[k].y;
                              ^~~
/in/foo.cc:53:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^