class Node:
  def __init__(self, v, l, r):
    self.v = v
    self.l = l
    self.r = r

t4 = Node(4, None, None);
t5 = Node(5, None, None);
t2 = Node(2, t4, t5);
t6 = Node(6, None, None);
t7 = Node(7, None, None);
t3 = Node(3, t6, t7);
t1 = Node(1, t2, t3);
stack = [("v", t1)]

while len(stack) >= 1:
  (x, top) = stack.pop()
  if (x == "v"):
    if top.r != None: stack.append(("v", top.r))
    stack.append ((("p", top)))
    if top.l != None: stack.append(("v", top.l))
  if (x == "p"):
    print top.v