Average of Levels in Binary Tree

https://leetcode.com/problems/average-of-levels-in-binary-tree/description/

Given a non-empty binary tree, return the average value of the nodes on each level in the form of an array.

Thoughts

看到按层遍历自然想到BFS, 用一个queue实现。

Code

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Double> averageOfLevels(TreeNode root) {
        List<Double> avgs = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        while (!queue.isEmpty()) {
            int length = queue.size();
            Double sum = 0.0;
            for (int i = 0; i < length; i++) {
                TreeNode node = queue.remove();
                sum += node.val;
                if (node.left != null) {
                    queue.add(node.left);
                }
                if (node.right != null) {
                    queue.add(node.right);
                }
            }
            avgs.add(sum / length);
        }

        return avgs;
    }
}

Analysis

时间复杂度和空间复杂度都是O(n)