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# 222. Count Complete Tree Nodes

## 題目 :

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 2h nodes inclusive at the last level h.

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

### Example :

Note

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

#### 解題思路 :

• 這題Recursive解非常簡潔且易懂~
• 每經過一個node便+1

### Recursive解法

• time complexity: O(n) , space complexity: O(1)

Runtime: 12 ms, faster than 99.33% of Go online submissions for Count Complete Tree Nodes.

 `````` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 `````` ``````/** * Definition for a binary tree node. * type TreeNode struct { * Val int * Left *TreeNode * Right *TreeNode * } */ func countNodes(root *TreeNode) int { if root == nil { return 0 } return 1 + countNodes(root.Left) + countNodes(root.Right) } ``````