[Leetcode] 637. 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.

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.

/**
 * 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> test = new LinkedList<>();
        test.add(root);
        while (!test.isEmpty()){
            int num = test.size();
            double sum = 0.0;
            for (int i = 0; i < num; i++) {
                TreeNode x = test.poll();
                sum = sum + x.val;
                if (x.left != null)
                    test.add(x.left);
                if (x.right != null)
                    test.add(x.right);                               
            }
            result.add(sum/num);
        }
        return result;
    }
}

p.s. 这里使用了queue来统计每一层element的个数！