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.

實現：
class Solution {
public:
int getHeight(TreeNode* root) {
int h = 0;
while (root) {
root = root->left;
h++;
}
return h;
}
int countNodes(TreeNode* root) {
if (!root) return 0;
int lh = getHeight(root->left);
int rh = getHeight(root->right);

if (lh == rh)
return pow(2, lh) + countNodes(root->right);
return pow(2, rh) + countNodes(root->left);
}
};

LeetCode222——Count Complete Tree Nodes