Breadth-first

Also, listed below is pseudocode for a simple queue based level order traversal, and will require space proportional to the maximum number of nodes at a given depth. This can be as much as the total number of nodes / 2. A more space-efficient approach for this type of traversal can be implemented using an iterative deepening depth-first search.

levelorder(root)
  q = empty queue
  q.enqueue(root)
  while not q.empty do
    node := q.dequeue()
    visit(node)
    if node.left ≠ null then
      q.enqueue(node.left)
    if node.right ≠ null then
      q.enqueue(node.right)

Breadth-first

原文：http://www.cnblogs.com/hephec/p/4603721.html