# Count Complete Tree Nodes Problem

## Description

LeetCode Problem 222.

Given the 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 1 and 2^h nodes inclusive at the last level h.

Design an algorithm that runs in less than O(n) time complexity.

Example 1:

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Input: root = [1,2,3,4,5,6]
Output: 6

Example 2:

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Input: root = []
Output: 0

Example 3:

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Input: root = [1]
Output: 1

Constraints:

- The number of nodes in the tree is in the range [0, 5 * 10^4].
- 0 <= Node.val <= 5 * 10^4
- The tree is guaranteed to be complete.

## Sample C++ Code

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class Solution {
public:
int countNodes(TreeNode* root) {
if(!root)
return 0;
int hl=0, hr=0;
TreeNode *l=root, *r=root;
while (l) {
hl ++;
l = l->left;
}
while (r) {
hr ++;
r = r->right;
}
if (hl==hr)
return pow(2,hl)-1;
return 1 + countNodes(root->left) + countNodes(root->right);
}
};