Basic Tree Traversal
Depth First Traversal:
1. In order traversal----left, root, right----4,2,5,1,3
2. Pre order traversal-----root, left, right-----1,2,4,5,3
3. Post order traversal----left,root,right----4,5,2,3,1
Breadth First Traversal (level order)
Post order traversal used at tree deletion
原文：http://www.cnblogs.com/superbo/p/4125535.html