[leetcode] Count Complete Tree Nodes

From : https://leetcode.com/problems/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.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int leftLevel(TreeNode* node) {
        if(!node) return 0;
        int level = 1, num;
        while(node->left) {
            level++;
            node = node->left;
        }
        return level;
    }
    int rightLevel(TreeNode* node) {
        if(!node) return 0;
        int level = 1, num;
        while(node->right) {
            level++;
            node = node->right;
        }
        return level;
    }
    int countNodes(TreeNode* root) {
        if(!root) return 0;
        int ll = leftLevel(root);
        int rl = rightLevel(root);
        if(ll == rl) return (1<<ll)-1;
        return 1 + countNodes(root->left) + countNodes(root->right);
        
    }
};