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 ( << leftDepth) - ;
         }
         return  + countNodes(root.left) + countNodes(root.right);

     }

     private int rightDepth(TreeNode root) {
         int depth = ;
         while (root != null) {
             root = root.right;
             depth++;
         }
         return depth;
     }

     private int leftDepth(TreeNode root) {
         int depth = ;
         while (root != null) {
             root = root.left;
             depth++;
         }
         return depth;
     }
 }