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.

/**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
int countNodes(TreeNode* root) {
if(root==NULL)
return 0;
TreeNode* tmp=root->left;
int leftheight=1;
int rightheight=1;
while(tmp!=NULL)
{
tmp=tmp->left;
leftheight++;
}
tmp=root->right;
while(tmp!=NULL)
{
tmp=tmp->right;
rightheight++;
}
if(leftheight==rightheight)
{
return (1<<leftheight)-1;
}
else
return countNodes(root->left)+countNodes(root->right)+1;
}
};