Problem
Write a function that will traverse a binary tree level by level. That is, the root level-by-level traversal is visited first, then the immediate children of the root, then the grandchildren of the root, and so on. [Hint: Use a queue to keep track of the children of a node until it is time to visit them. The nodes in the first tree of Figure 10.7 are numbered in level-by-level ordering.]
