leetcode-637-easy

Average of Levels in Binary Tree

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,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:

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

思路一：数的层次遍历，使用队列实现，执行效率不太行

    public List<Double> averageOfLevels(TreeNode root) {
        List<Double> ans = new ArrayList<>();

        Queue<TreeNode> queue = new ArrayDeque<>();
        queue.offer(root);

        while (!queue.isEmpty()) {
            List<TreeNode> nodes = new ArrayList<>();
            double sum = 0;
            while (!queue.isEmpty()) {
                TreeNode poll = queue.poll();
                nodes.add(poll);
                sum += poll.val;
            }

            ans.add(sum / nodes.size());

            for (TreeNode node : nodes) {
                if (node.left != null) {
                    queue.offer(node.left);
                }

                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
        }

        return ans;
    }