思路

层次遍历

class Solution {
    public List<Double> averageOfLevels(TreeNode root) {
        List<Double> ans = new ArrayList<>();
        if(root==null)
            return ans;
        Queue<TreeNode> queue = new ArrayDeque<>();
        queue.offer(root);
        while(!queue.isEmpty()){
            int count = queue.size();
            int num = count;
            double total = 0;
            while(count-->0){
                TreeNode node = queue.poll();
                if(node==null)
                    continue;
                total+=node.val;
                if(node.left!=null)
                    queue.offer(node.left);
                if(node.right!=null)
                    queue.offer(node.right);
            }
            ans.add(total/num);
        }
        return ans;
    }
}