[数据结构与算法]lc637——二叉树的层平均值

?思路：在层序遍历的时候把每层求个总和在求平均

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Double> averageOfLevels(TreeNode root) {
     //返回的结果
        List<Double> res = new ArrayList<>();
        
        if(root == null){
            return res;
        }
        
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        
        //根节点入队
        queue.add(root);
        
        while(!queue.isEmpty()){
         
            int levelCount = queue.size();
            double n=0.0;
            //添加节点到一层的List中去
            for(int i = 0; i < levelCount ; i ++){
                //节点出队
                TreeNode node = queue.remove();
                n=n+node.val;
                //节点的左子树入队
                if(node.left != null){
                  queue.add(node.left);   
                }
 
                //节点的右子树入队   
                if(node.right != null){
                  queue.add(node.right);   
                }  
            }
                res.add(n/levelCount);
            
        }
      return res;
}
  
}