【Count Complete Tree Nodes】cpp

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.

代码：

/**
 * 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 hl = 1;
            TreeNode* l = root->left;
            int hr = 1;
            TreeNode* r = root->right;
            while ( l ){
                ++hl;
                l = l->left;
            }
            while ( r ){
                ++hr;
                r = r->right;
            }
            if ( hr==hl ) return pow(2, hr)-1;
            return Solution::countNodes(root->left) + Solution::countNodes(root->right) + 1;
    }
};

tips：

学习大神的递归代码：http://bookshadow.com/weblog/2015/06/06/leetcode-count-complete-tree-nodes/

（1）如果left height=right height则是full tree可以直接返回节点数

（2）如果left height!=right height则不是full tree，分别递归求解left和right的节点数