data test;
   input x @@;
cards;
1 1 2 3 1 2 1 2 1 2 1 1 2 3 4 5 6
;
run;

data want;
   set test;
   id=_n_;
run;

proc sort data=want;
   by descending id;
run;

data want;
   set want;
   retain y;
   dif=dif(x);
   if _n_=1 then y=x;
   else if dif>=0 then y=x;
   drop dif;
run;

proc sort data=want;
   by id;
run;

proc print;run;

谢谢大神