Count Complete Tree Nodes

https://leetcode.com/problems/count-complete-tree-nodes

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

Note:

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.

Example:

Input: 
    1
   / \
  2   3
 / \  /
4  5 6

Output: 6

Implementation :

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int totalNodes = 0;
    public int countNodes(TreeNode root) {
        if(root == null)
            return 0;
        helper(root);
        return totalNodes;
    }
    
    public void helper(TreeNode root){
        if(root != null){
            totalNodes++;
            helper(root.left);
            helper(root.right);
        }
    }
}