tags: leetcode medium Binary Search Tree Binary Tree Depth_first Search

222. Count Complete Tree Nodes

Description

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.

Examples

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 $[0, 5 * 10^4]$.
• $0\leq Node.val\leq 5\times 10^4$
• The tree is guaranteed to be complete.

Code

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
int countNodes(struct TreeNode* root){
    if (root == NULL)
        return 0;
    
    if(!root->left && !root->right)
        return 1;
    
    return 1 + countNodes(root->left) + countNodes(root->right);
}

Complexity

Space	Time
$O(1)$	$O(n)$

Result

• Runtime: 46 ms, faster than 59.66%
• Memory Usage: 16.8 MB, less than 70.00%

Notes