動機

複習bfs

Problem

Given the root of a binary tree, return the average value of the nodes on each level in the form of an array. Answers within 10-5 of the actual answer will be accepted.

 

Example 1:

Input: root = [3,9,20,null,15,7]Output: [3.00000,14.50000,11.00000]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].

Example 2:

<img alt="" src=“https://assets.leetcode.com/uploads/2021/03/09/avg2-tree.jpg “style=“width: 292px; height: 302px;” />
Input: root = [3,9,20,15,7]Output: [3.00000,14.50000,11.00000]

 

Constraints:

  • The number of nodes in the tree is in the range [1, 104].
  • -231 <= Node.val <= 231 - 1

Sol

class Solution:
    def averageOfLevels(self, root: TreeNode) -> List[float]:
        q = deque([[root]])
        ret = []
        while q:
            rs = q.popleft()
            ret.append(sum([r.val for r in rs])/len(rs))
            next_rs = sum([[r.left, r.right] for r in rs],[])
            next_rs = [r for r in next_rs if r]
            if next_rs:
                q.append(next_rs)
        return ret