How to convert a tree recursive function ( or algorithm ) to a loop one?

I have written a recursive Tree Function in pascal ( or delphi ) but i had an 'Out of Memory' message when I ran it. I need to turn the Calculate recursive function in this code to non-recursive function, can you tell me how please :

program testing(input, output);
type
  ptr = ^tr;
  tr = record
    age: byte;
    left, right: ptr;
  end; 

var
  topper: ptr;
  total, day: longint;

  procedure mycreate(var t: ptr);
  var 
    temp:ptr;

  begin
    new(temp);
    temp^.age := 1;
    temp^.left := nil;
    temp^.right := nil;
    t := temp;

  end;

  procedure gooneday(var t: ptr);
  begin
    if t^.age <> 5 then
    begin
      if t^.age = 2 then
        mycreate(t^.left)
      else if t^.age = 3 then
        mycreate(t^.right);
      
      t开发者_JAVA技巧^.age := t^.age + 1;
      total := total + 1;
    end;

  end;

  procedure calculate(var crnt: ptr);
  begin
    if crnt <> nil then
    begin
      gooneday(crnt);
      calculate(crnt^.left);
      calculate(crnt^.right);
    end;

  end;

begin
  total := 0;
  mycreate(topper);
  day := 0;

  while total < 1000000000000 do
  begin
    total := 0;
    day := day + 1;
    calculate(topper);
  end;

  writeln(day);
  writeln(total);

end.

Recursive functions use a stack to keep the state of the recursion.

When converting to a loop, you must actually create an explicit stack. You must push and pop elements off the stack within the loop.