Leetcode: 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.

Code:

/** * 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 countDepth(TreeNode* root, int dir) {        int ret = 0;        while (root) {            ++ret;            if (dir == 0)                root = root->left;            else                 root = root->right;        }        return ret;    }    int countNodes(TreeNode* root) {        if (root == NULL) {            return 0;        } else if (root->left == NULL) {            return 1;        } else if (root->right == NULL) {            return 2;        } else {            int d0, d1;            d0 = countDepth(root, 0);            d1 = countDepth(root, 1);            if (d0 == d1) {                return pow(2, d0) - 1;            }            return countNodes(root->left) + countNodes(root->right) + 1;        }    }};