# Constructor to create a new node
def __init__(self, data):
    self.data = data
    self.left = None
    self.right = None

# Set current to root of binary tree
current = root
stack = [] # initialize stack
done = 0
 
while True:
     
    # Reach the left most Node of the current Node
    if current is not None:
         
        # Place pointer to a tree node on the stack
        # before traversing the node's left subtree
        stack.append(current)
     
        current = current.left

     
    # BackTrack from the empty subtree and visit the Node
    # at the top of the stack; however, if the stack is
    # empty you are done
    elif(stack):
        current = stack.pop()
        print(current.data, end=" ") # Python 3 printing
     
        # We have visited the node and its left
        # subtree. Now, it's right subtree's turn
        current = current.right

    else:
        break
  
print()