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 2^h nodes inclusive at the last level h.

if countLeft == countRight, return count(countLeft)

else return countNodes(root->left) + countNodes(root->right) + 1;

/**
 * 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 == nullptr) return 0;
        int left = countLeftLevel(root);
        int right = countRightLevel(root);
        if(left == right){
            return count(left);
        }
        else{
            return countNodes(root->left) + countNodes(root->right) + 1;
        }
    }
    int countLeftLevel(TreeNode* root){
        int count = -1;
        while(root){
            count++;
            root = root->left;
        }
        return count;
    }
    int countRightLevel(TreeNode* root){
        int count = -1;
        while(root){
            count++;
            root = root->right;
        }
        return count;
    }
    int count(int n){
        int sum = 1;
        int factor = 2;
        while(n--){
            sum += factor;
            factor *= 2;
        }
        return sum;
    }
};