代码:

    高精度加法
function hiPlus(s1,s2:string):string;
var a,b:array[1..100] of byte;
    k,l1,l2,i,j:integer;tmpstr:string;
begin
  l1:=length(s1);l2:=length(s2);
  fillchar(a,sizeof(a),0);
  fillchar(b,sizeof(b),0);
  if l1>l2 then j:=l1 else j:=l2;
  k:=0;
  for i:=l1 downto 1 do
    begin
      inc(k);
      a[k]:=ord(s1[i])-48;
    end;
  k:=0;
  for i:=l2 downto 1 do
    begin
      inc(k);
      b[k]:=ord(s2[i])-48;
    end;
  for i:=1 to j do
    begin [
      a[i]:=a[i]+b[i];
      if a[i]>=10 then
        begin
          a[i]:=a[i]-10;
          a[i+1]:=a[i+1]+1;
        end; ]
    end;
  if a[j+1]=0 then j:=j-1;
  tmpstr:='';
  for i:=j+1 downto 1 do
    tmpstr:=tmpstr+chr(a[i]+48);
  hiPlus:=tmpstr
end;  
    高精度减法
if a[i]<b[i] then
  begin
    a[i+1]:=a[i+1]-1;
    a[i]:=a[i]+10;
  end;
a[i]:=a[i]-b[i];