Count Complete Tree Nodes

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

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.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int countNodes(TreeNode root) {
    
        int leftDepth = leftDepth(root);
        int rightDepth = rightDepth(root);
    
        if (leftDepth == rightDepth) {
            return (1 << leftDepth) - 1;
        }
        return 1 + countNodes(root.left) + countNodes(root.right);
    
    }
    
    private int rightDepth(TreeNode root) {
        int depth = 0;
        while (root != null) {
            root = root.right;
            depth++;
        }
        return depth;
    }
    
    private int leftDepth(TreeNode root) {
        int depth = 0;
        while (root != null) {
            root = root.left;
            depth++;
        }
        return depth;
    }
}