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# 222. Count Complete Tree Nodes
## Problem Statement
Given the `root` of a **complete** binary tree, return the number of the nodes in the tree.
According to [**Wikipedia**](http://en.wikipedia.org/wiki/Binary_tree#Types_of_binary_trees), 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 `2h` nodes inclusive at the last level `h`.
Design an algorithm that runs in less than `O(n)` time complexity.
**Example 1:**
Input: root = [1,2,3,4,5,6]
Output: 6
**Example 2:**
Input: root = []
Output: 0
**Example 3:**
Input: root = [1]
Output: 1
**Constraints:**
* 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**.
## Intuition
```
Approach 1:
Traversal
Approach 2:
Nodes in complete BT is 2^n-1;
so we travel all the way to left and right
if found equal, return 2^n-1
Else we move left and right
Now the complexity of this is logn*logn
As in the worst case, We are going to logn nodes for finding l == r condition
And at all logn nodes we find height for l and r in logn time
Hence logn * logn
```
### Links
### Video Links
### Approach 1:
```
Brute
```
{% code title="C++" lineNumbers="true" %}
```cpp
class Solution {
public:
int countNodes(TreeNode* root) {
if(root == nullptr)
return 0;
if(!root->left and !root->right)
return 1;
int l=0,r=0;
if(root->left)
l = countNodes(root->left);
if(root->right)
r = countNodes(root->right);
return 1 + l + r;
}
};
```
{% endcode %}
### Approach 2:
```
Optimal
```
{% code title="C++" lineNumbers="true" %}
```cpp
class Solution {
public:
int countNodes(TreeNode* root) {
if(root == nullptr)
return 0;
int l=0, r=0;
TreeNode *lptr=root, *rptr=root;
while(lptr){
lptr = lptr->left;
l++;
}
while(rptr){
rptr = rptr->right;
r++;
}
if(l == r)
return (1<left) + countNodes(root->right);
}
};
```
{% endcode %}
### Approach 3:
```
```
{% code title="C++" lineNumbers="true" %}
```cpp
```
{% endcode %}
### Approach 4:
```
```
{% code title="C++" lineNumbers="true" %}
```cpp
```
{% endcode %}
### Similar Problems
###
---
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