637. Average of Levels in Binary Tree

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   7Output: [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.

思路： BFS

Java代码如下：

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */import java.util.*;public class Solution {    public List<Double> averageOfLevels(TreeNode root) {        List<Double> result = new LinkedList<>();        List<TreeNode> list = new LinkedList<>();        list.add(root);        while(!list.isEmpty()) {            int size = list.size();            double sum = 0.0;            for(int i = 0; i < size; i++) {                TreeNode node = list.get(0);                sum += node.val;                 if(node.left != null) {                    list.add(node.left);                }                if(node.right != null) {                    list.add(node.right);                }                list.remove(node);            }            double avg = sum / size;             result.add(avg);        }        return result;    }}