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题目：
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.
解答1，（错误）
/** * 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 == NULL) return 0; // int leftHeight = 0; // int rightHeight = 0; // TreeNode *pLeft = root; // while(pLeft) { // ++leftHeight; // pLeft = pLeft->left; // } // TreeNode *pRight = root; // while(pRight) { // ++rightHeight; // pRight = pRight->right; // } // if(leftHeight == rightHeight) // return pow(2, leftHeight)-1; return countNodes(root->left) + countNodes(root->right) + 1; } };
解答，正确：
把上面的中间代码的反注释掉，运行通过。
心得：
这个题目，需要利用到满二叉树的节点数为2的N次方减1，并且我们做题目的时候，通常做出来的是前面一种解答的形式。
这个题目，混搭了满二叉树的求节点数，和完全二叉树的性质。当然，我也只做到了前面一种解答，测试代码报告效率不高，时间已经超过了最大时间。后面再补上了中间的代码。
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原文地址：http://www.cnblogs.com/ender-cd/p/4617126.html
