LeetCode-Average of Levels in Binary Tree

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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:

• The range of node’s value is in the range of 32-bit signed integer.

题意：计算一颗二叉树的每一层所有节点值相加所得的平均数；

解法：实际上我们需要解决的是二叉树的层次遍历问题；我们可以利用队列来实现，首先，将根节点入队；每次遍历队列中所有元素，这些元素都是这一层的所有节点，可以求得节点值得平均值，对这些节点遍历时，同时将其所有节点入队；直到最后队列为空；

Java

/**
 * 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<>();
        LinkedList<TreeNode> nodes = new LinkedList<>();
        if (root == null) return result;
        nodes.add(root);
        while (nodes.size() > 0) {
            double sum = 0L;
            int len = nodes.size();
            for (int i = 0; i < len; i++) {
                TreeNode temp = nodes.remove();
                sum += temp.val;
                if (temp.left != null) nodes.add(temp.left);
                if (temp.right != null) nodes.add(temp.right);
            }
            result.add(sum / len);
        }
        return result;
    }
}