637. Average of Levels in Binary Tree

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:

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

以数组的形式返回每个层次上的节点的平均值

C++（19ms）： bfs

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<double> averageOfLevels(TreeNode* root) {
13         if (root == NULL)
14             return vector<double>() ;
15         vector<double> res ;
16         queue<TreeNode*> que ;
17         que.push(root) ;
18         while(!que.empty()){
19             long long sum = 0 ;
20             int len = que.size() ;
21             for(int i = 0 ; i < len ; i++){
22                 TreeNode* t = que.front() ;
23                 que.pop() ;
24                 sum += t->val ;
25                 if (t->left != NULL)
26                     que.push(t->left) ;
27                 if (t->right != NULL)
28                     que.push(t->right) ;
29             }
30             res.push_back(sum/(len*1.0));
31         }
32         return res ;
33     }
34 };