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:

  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 averageOfLevels(TreeNode root) {
        List result = new LinkedList();
        List 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;
    }
}
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