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 One: 

Input: root = [1,2,3,4,5,6]
Output: 6

Example Two:

Input: root = []
Output: 0

 Solution:

class Solution {
    List<Integer> result = new ArrayList<>();
    public int countNodes(TreeNode root) {
        helper(root, 0);
        return result.size();
    }
    public void helper(TreeNode node, int level){
        if(node == null) return ;
        // it is not necessary to have these two clauses
        //List<Integer> levelList = new ArrayList<>();
        //if(result.size() == level)result.add(levelList);
        result.add(node.val);

        if(node.left != null) helper(node.left, level + 1);
        if(node.right != null) helper(node.right, level + 1);
        
    }
}

 Hints:

Logics:

• know how to finish the framework of the code
• use BFS (Breath-First-Search)
• add each value to the list 
• count the size of the list