program de;
    var
    a:array[0..100000] of longint;
    b:array[0..100000] of word;
    c:array[0..1000] of longint;
    i,j,n,x,y,z:longint;
    code:integer;
    s:string;
begin
    readln(n);
    for i:=1 to n do
     begin
       readln(s);
       if (pos('PUSH',s)<>0) then
        begin
         inc(z);
         delete(s,1,5);
         for j:=1 to length(s) do if s[j]=' ' then break;
         val(copy(s,1,j-1),x,code);
         val(copy(s,j+1,length(s)-j),y,code);
         inc(z);
         a[z]:=y;
         b[z]:=c[x];
         c[x]:=z;
        end
       else
         begin
          delete(s,1,4);
          val(s,x,code);
          writeln(a[c[x]]);
          c[x]:=b[c[x]];
         end;
     end;
end.

I made one change....and got TLE....
var
a:array[0..100000] of longint;
b:array[0..100000] of word;
c:array[0..1000] of longint;
i,j,n,x,y,z:longint;
code:integer;
s:string;
begin
readln(n);
for i:=1 to n do
begin
readln(s);
if (pos('PUSH',s)<>0) then
begin
inc(z);
delete(s,1,pos('PUSH',s)+4);
for j:=1 to length(s) do if s[j]=' ' then break;
val(copy(s,1,j-1),x,code);
val(copy(s,j+1,length(s)-j),y,code);
inc(z);
a[z]:=y;
b[z]:=c[x];
c[x]:=z;
end
else
begin
delete(s,1,pos('POP',s)+3);
val(s,x,code);
writeln(a[c[x]]);
c[x]:=b[c[x]];
end;
end;
end.