637. Average of Levels in Binary

Given a non-empty binary tree, return the average value of the nodes on each level in the form of an array.
树的BFS遍历 没啥好说的

/**
 * 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) {
        ArrayList<Double> result =new ArrayList<>();
        ArrayDeque<TreeNode> queue = new ArrayDeque<>();
        if(root!=null)
        queue.add(root);
        else
        return result;
        while(!queue.isEmpty())
        {
            int count = queue.size();
            double sum = 0;
            for(int i = 0;i<count;i++)
            {
                TreeNode node = queue.poll();
                sum+=node.val;
                if(node.left!=null)
                    queue.add(node.left);
                if(node.right!=null)
                    queue.add(node.right);
                
            }
            result.add(sum/count);
        }
        return result;
    }
}