# Time:  O(n)
# Space: O(h)

# 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.

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

class Solution(object):
    def averageOfLevels(self, root):
        """
        :type root: TreeNode
        :rtype: List[float]
        """
        result = []
        q = collections.deque([root])
        while q:
            total, count = 0, 0
            next_q = collections.deque([])
            while q:
                n = q.popleft()
                total += n.val;
                count += 1
                if n.left:
                    next_q.append(n.left)
                if n.right:
                    next_q.append(n.right)
            q, next_q = next_q, q
            result.append(float(total) / count)
        return result

Create average-of-levels-in-binary-tree.py