Pre order traversal of a tree

pre_order_traversal.py

def pre_order_recursive(input, index):
    if index >= len(input) or input[index] == None:
        return
    print input[index],
    pre_order_recursive(input, (index * 2) + 1)
    pre_order_recursive(input, (index * 2) + 2)

pre_order_recursive([3,2,1,1,1,1,0,1,0], 0)
# 3 2 1 1 0 1 1 1 0

def pre_order(input):
    stack = []
    if input is None:
        return
    stack.append(0)
    while len(stack) > 0:
        current = stack.pop()
        print input[current], 
        left = (current * 2) + 1
        right = (current * 2) + 2
        if right < len(input) and input[right] != None:
            stack.append(right)
        if left < len(input) and input[left] != None:
            stack.append(left)

pre_order([3,2,1,1,1,1,0,1,0])
# 3 2 1 1 0 1 1 1 0