Input: Postfix expression E, output: Expression tree T
 initialize stack S;
 for j = 0 to E.size – 1 do
   if (E[j] is an operand) {
    Tree t = new Tree(E[j]);
    S.push(t*);}
   else {
     tree* t2 = S.pop();
     tree* t1 = S.pop();
     Tree t = new(E[j], t1, t2);
     S.push(t*);
   }