old_contents: # 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
              result.append(float(total) / count)
          return result
  
new_contents: # 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, next_q = collections.deque([root]), collections.deque([])
          while q:
              total, count = 0, 0
              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
  
subject: Update average-of-levels-in-binary-tree.py