A Complete Binary Tree （CBT) is a binary tree in which every level, 
except possibly the last, is completely filled, and all nodes 
are as far left as possible.

换句话说，完全二叉树从根结点到倒数第二层满足完美二叉树，最后一层可以不完全填充，其叶子结点都靠左对齐。

例如：