root of a complete binary tree, return the number of the nodes in the tree.
According to Wikipedia, every level, except possibly the last, is completely filled in a complete binary tree, and all nodes in the last level are as far left as possible. It can have between
2h nodes inclusive at the last level
Design an algorithm that runs in less than
O(n) time complexity.
1 / \ 2 3 / \ / 4 5 6 Input: root = [1,2,3,4,5,6] Output: 6
Input: root =  Output: 0
Input: root =  Output: 1
- The number of nodes in the tree is in the range
[0, 5 * 104].
0 <= Node.val <= 5 * 104
- The tree is guaranteed to be complete.