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 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:
The range of node’s value is in the range of 32-bit signed integer.
題意：求二叉樹每一層的平均值。

思路：藉助隊列Queue，層序遍歷

/**
 * 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> result = new ArrayList<>();
        Queue<TreeNode> queue=new LinkedList<>();
        queue.offer(root);

        //double sum=0;
        while(!queue.isEmpty()){
            int n =queue.size();//size in each level
            double sum=0.0;
            for(int i=0;i<n;i++){
                TreeNode node=queue.poll();
                sum+=node.val;
                if(node.left!=null) queue.offer(node.left);
                if(node.right!=null) queue.offer(node.right);
        } 
            result.add(sum/n);
        }    
        return result;
    }
}