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;
    }
};