# Print Binary Tree Layer-by-layer

## Question

Print a binary tree layer-by-layer from top to bottom, and from left to right for each layer.

## Solution

Yes, it’s a simple task. We can use breadth-first search, and which means we need a queue.

## Example

```
# node structure
class bst_node:
def __init__(self, value, left=None, right=None):
self.value = value
self.left = left
self.right = right
# what to do with a node.
def visit(node):
print node.value, # bfs traversal
def breadth_first_traversal(root):
queue = [root]
while len(queue) > 0:
node = queue.pop(0) # get the head item
visit(node)
if node.left: queue.append(node.left)
if node.right: queue.append(node.right)
# a binary tree and display how was it before reflection
node_05 = bst_node(5)
node_07 = bst_node(7)
node_09 = bst_node(9)
node_11 = bst_node(11)
node_06 = bst_node(6, node_05, node_07)
node_10 = bst_node(10, node_09, node_11)
node_08 = bst_node(8, node_06, node_10)
root = node_08
# answer should be: 8 6 10 5 7 9 11
breadth_first_traversal(root)
```