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 2hnodes inclusive at the last level h.

题目意思是给一个完全二叉树，返回节点的总个数，完全二叉树肯定是上一层是满二叉树，只可能最后一层没满，而对于满二叉树直接用2^n-1就可以求到，刚开始想的直接用递归求，结果超时，然后想用队列遍历一遍，还是超时，最后，看了下别人怎么做的。就是直接用那个性质，每次先求到左右子树的高度，如果一样，则用公式求，不一样则递归。

/** * 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) return 0;int leftheight = 0, rightheight = 0;TreeNode* nleft = root;TreeNode* nright = root;while (nleft)//一直向左{leftheight++;nleft = nleft->left;}while (nright)//一直向右{rightheight++;nright = nright->right;}if (leftheight==rightheight)//是完全二叉树直接按公式算{return pow(2, leftheight) - 1;}return countNodes(root->left) + countNodes(root->right) + 1;}};