Leetcode 637. Average of Levels in Binary Tree

题目链接

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

题目描述

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

Example 1:

Input:
    3
   / 
  9  20
    /  
   15   7
Output: [3, 14.5, 11]
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].

Note:

1. The range of node's value is in the range of 32-bit signed integer.

题解

依次遍历每一层，统计结果并求出平均值。

代码

/**
 * 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> list = new ArrayList<>();
        if (root == null) { return list; }
        LinkedList<TreeNode> s = new LinkedList<>();
        s.offer(root);
        while (!s.isEmpty()) {
            int size = s.size();
            double sum = 0;
            for (int i = 0; i < size; i++) {
                TreeNode node = s.poll();
                if (node.left != null) { 
                    s.offer(node.left);
                }
                if (node.right != null) {
                    s.offer(node.right);
                }
                sum += (double)node.val;
            }
            list.add(sum / size);
        }
        return list;
    }
}