Given the root node root of a complete binary tree, return the total number of nodes in the tree.

A complete binary tree is a binary tree where every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible.

Note: The input tree is guaranteed to be complete.

Constraints:

• The number of nodes in the tree is in the range $[0, 5 \cdot 10^4]$.

• $0 \leq$ Node.val $\leq 5 \cdot 10^4$.

• The tree is complete.

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