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222. Count Complete Tree Nodes

MediumOpen on LeetCodeProblem statement

Problem Statement

222. Count Complete Tree Nodes

Medium

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 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:

C++

Source file
class Solution {
public:
    int countNodes(TreeNode* root) {
        if(!root)   return 0;
        return 1+countNodes(root->left)+countNodes(root->right);
    }
};