leetcode 637. 二叉树的层平均值

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Double> averageOfLevels(TreeNode root) {
        List<Double> list = new ArrayList<>();
        if (root == null) {
            return list;
        }
        Queue<TreeNode> a = new ArrayDeque<>();
        Queue<TreeNode> b = new ArrayDeque<>();
        a.add(root);
        while (!a.isEmpty() ) {
            double sum = 0D;
            int count = 0;
            while (!a.isEmpty()) {
                TreeNode poll = a.poll();
                sum += poll.val;
                count++;
                if (poll.left != null) {
                    b.add(poll.left);
                }
                if (poll.right != null) {
                    b.add(poll.right);
                }
            }
            a = b;
            b = new ArrayDeque<>();
            list.add(sum / count);
        }
        return list;
    }
}