Используя Паскаль:
var rndm: array[1..20] of longint;
i,n1.n2,c : longint;
begin
for i:=1 to 20 do rndm[i]:=i;
for i:=1 to 100 do begin
n1:=random(19)+1;
n2:=random(19)+1;
c:=rndm[n1];
rndm[n1]:=rndm[n2];
rndm[n2]:=c;
end;
for i:=1 to 20 do write (rndm[i],' ');
end.