[Leetcode]222. Count Complete Tree Nodes

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Given a complete binary tree, count the number of nodes.
Note: 
Definition of a complete binary tree from Wikipedia:
In a complete binary tree every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2h nodes inclusive at the last level h.
Example:
Input: 
    1 
   / \ 
  2   3 
 / \ / 
4 5 6 
Output: 6
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