## [Leetcode]222. 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
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## Tree Traversal – Recursively & Iteratively

Tree traversal refers to the process of visiting each node in a tree data structure (Wikipedia). The two general strategies are Depth-First-Search (DFS) and Breadth-First-Search (BFS). For BFS, it iterates through the tree level by level with Queue, from top to bottom. When using DFS, there are three different ways: Preorder, Inorder, and Postorder.

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