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.

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;
}
}