leetcode讲解--637. Average of Levels in Binary Tree

题目

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.

讲解

这题跟429. N-ary Tree Level Order Traversal很像。都是广度优先遍历树（也就是按层级遍历树）。

java代码

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List result = new ArrayList<>();
    public List averageOfLevels(TreeNode root) {
        if(root==null){
            return result;
        }
        Queue queue = new LinkedList<>();
        queue.offer(root);
        int children_num=1;
        while(!queue.isEmpty()){
            double sum = 0;
            int count=0;
            for(int i=0;i