LeetCode - 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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int countNodes(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int mostLeftLevel = 1;
        int  mostRightLevel = 1;
        TreeNode left = root.left;
        TreeNode right = root.right;
        while (left != null) {
            mostLeftLevel++;
            left = left.left;
        }
        while(right != null) {
            mostRightLevel++;
            right = right.right;
        }
        if (mostLeftLevel == mostRightLevel) {
            return (int)Math.pow(2, mostLeftLevel) - 1;
        }
        return 1 + countNodes(root.left) + countNodes(root.right);
        
    }
}