def dfs_stack(root, fn):
stack = Stack()
stack.push(root)
visited = defaultdict(bool)
visited[root] = True
fn(root)
while not stack.is_empty():
node = stack.get_top()
for child in node.get_children():
if not visited.get(child):
fn(child)
visited[child] = True
stack.push(child)
else:
stack.pop()
