222. Count Complete Tree Nodes

https://leetcode.com/problems/count-complete-tree-nodes/

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