leetCode 637. Average of Levels in Binary Tree 求二叉树每层平均值

题目描述：

Given a non-empty binary tree, return the average value of the nodes on each level in the form of an array.

Example 1:

Input :
        3
      /    \
    9      20
          /    \
        15      7
Output : [ 3 , 14.5, 11 ]
Explanation :
The average value of nodes on level 0 is 3, on level 1 is 14.5, and on level 2 is 11. Hence return [3, 14.5, 11].

Note:

　１.The range of node’s value is in the range of 32-bit signed integer.

解题思路：

求解每层节点的平均值，“层”第一时间应该想到的是层次遍历（level-traverse)；遍历每一层的节点求出平均值，并将该层的子节点存入数组。

c++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:  vector<double> averageOfLevels(TreeNode* root) { vector<double> result; vector<TreeNode *> vec; int lastL = 0; double average = 0; vec.push_back(root); while (lastL != vec.size()) {       int times = 0;     for (int i = lastL,size = vec.size(); i <size ; ++i,++times){         average += vec[i]->val;         if (vec[i]->left)             vec.push_back(vec[i]->left);         if (vec[i]->right)             vec.push_back(vec[i]->right);     }     result.push_back(average / times); average = 0;     lastL += times;   } return result; }};