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 2hnodes inclusive at the last level h.

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     int countNodes(TreeNode* root) {
13         if(root==NULL)
14             return 0;
15         TreeNode* tmp=root->left;
16         int leftheight=1;
17         int rightheight=1;
18         while(tmp!=NULL)
19         {
20             tmp=tmp->left;
21             leftheight++;
22         }
23         tmp=root->right;
24          while(tmp!=NULL)
25         {
26             tmp=tmp->right;
27             rightheight++;
28         }
29         if(leftheight==rightheight)
30         {
31             return (1<<leftheight)-1;
32         }
33         else
34             return countNodes(root->left)+countNodes(root->right)+1;
35     }
36 };