# Leetcode: Count Complete Tree Nodes

Count Complete Tree Nodes

Similar Problems:

Given a complete binary tree, count the number of nodes.

Definition of a complete binary tree from Wikipedia:
In a complete binary tree every level, except possibly the last, is completely filled, 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.

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.

```## Blog link: https://code.dennyzhang.com/count-complete-tree-nodes
## Basic Ideas: Cut the examine dataset into half
##              Check height of sub-tree and right-tree
##              For each divide-conquer, one half will be solved without recursive
##
##    Sample Data:
##             1
##           /   \
##          2     3
##         /
##        4
##
## Complexity: Time O(h*log(n)) = O(log(n)*log(n)). Space O(1). If include system stack, Space O(log(n))
# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
def countNodes(self, root):
"""
:type root: TreeNode
:rtype: int
"""
if root is None:
return 0

# root node is with height 0
lh, rh = 0, 0
lnode, rnode = root, root
while lnode:
lnode = lnode.left
lh += 1
while rnode:
rnode = rnode.right
rh += 1

if lh == rh:
return pow(2, lh) - 1
else:
return self.countNodes(root.left) + self.countNodes(root.right) + 1
```

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