Solution2.java

/**
 * 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 {
    private List<Long> s = new ArrayList<>();
    private List<Integer> cnt = new ArrayList<>();

    public List<Double> averageOfLevels(TreeNode root) {
        dfs(root, 0);
        List<Double> ans = new ArrayList<>();
        for (int i = 0; i < s.size(); ++i) {
            ans.add(s.get(i) * 1.0 / cnt.get(i));
        }
        return ans;
    }

    private void dfs(TreeNode root, int i) {
        if (root == null) {
            return;
        }
        if (s.size() == i) {
            s.add((long) root.val);
            cnt.add(1);
        } else {
            s.set(i, s.get(i) + root.val);
            cnt.set(i, cnt.get(i) + 1);
        }
        dfs(root.left, i + 1);
        dfs(root.right, i + 1);
    }
}