# Average of Levels in Binary Tree(BFS 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].
```

分析

bfs tree

```
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def averageOfLevels(self, root):
        """
        :type root: TreeNode
        :rtype: List[float]
        """
        q = collections.deque()
        q.append(root)
        ret = []
        while q:
            l = len(q)
            sum = 0
            for i in range(l):
                cur = q.popleft()
                sum += cur.val
                if cur.left:
                    q.append(cur.left)
                if cur.right:
                    q.append(cur.right)
            ret.append(sum/l)

        return ret
```
