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.

按层遍历，求每层平均值。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.LinkedList;
import java.util.Queue;
public class Solution {
    public List<Double> averageOfLevels(TreeNode root) {
        List<Double> retList = new ArrayList<Double>();
        if (root == null)
            return retList;

        Queue<TreeNode> q = new LinkedList<TreeNode>();
        q.offer(root);

        while ( ! q.isEmpty() ) {
            List<TreeNode> tl = new ArrayList<TreeNode>();
            double sum = 0;
            while ( ! q.isEmpty() ) {
                TreeNode curr = q.poll();
                tl.add(curr);
                sum += (double) curr.val;
            }
            retList.add(sum / tl.size());
            for (TreeNode n : tl) {
                if (n.left != null)
                    q.offer(n.left);
                if (n.right != null)
                    q.offer(n.right);
            }
        }
        return retList;
    }
}