Given the root of a binary tree, return the total number of nodes in the tree.
Each node in the tree contains a value and may have a left child, a right child, or both. The task is to count all nodes present in the tree, including the root.
Example 1
Input: root = [1,2,3,4,5,6]
Output: 6
Explanation: The tree contains six nodes: 1, 2, 3, 4, 5, and 6. Therefore, the total node count is 6.
Example 2
Input: root = [1,null,2]
Output: 2
Explanation: The tree has two nodes: 1 and 2. Hence, the total count is 2.
Example 3
Input: root = []
Output: 0
Explanation: If the tree is empty, there are no nodes, so the count is 0.
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