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.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.Queue;
class Solution {
    public List<Double> averageOfLevels(TreeNode root) {
        if (root == null)
            return null;
        List<Double> list = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        list.add((double)root.val);
        queue.add(root);
        TreeNode last = root;
        TreeNode nextLast = root;
        double averageSum  = 0;
        int count = 0;
        while (queue.size() != 0) {
            TreeNode temp = queue.poll();

            if (temp.left != null) {
                nextLast = temp.left;
                queue.add(temp.left);
                averageSum += temp.left.val;
                count ++;
            }

            if (temp.right != null) {
                nextLast = temp.right;
                queue.add(temp.right);
                averageSum += temp.right.val;
                count ++;
            }

            if (temp == last) {
                if (nextLast != last) {                    
                     list.add(averageSum / count);
                     averageSum = 0;
                     count = 0;
                     last = nextLast;
                }   
            }  
        }
        return list;

    }
}