LeetCode 637 : Average of Levels in Binary Tree（java）

原题：
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   7Output: [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].

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：求二叉树每一层的平均值。用leafNumList存储每一层叶子节点个数，用deepList存储每一层的平均值。

代码：

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public void calculation(TreeNode root,int deep,List<Double> deepList,List<Integer> leafNumList){        if(root!=null){            if(deepList.size()<deep){                deepList.add(1.0*root.val);                leafNumList.add(1);            }else{                deepList.set(deep-1,(deepList.get(deep-1)*leafNumList.get(deep-1)+root.val)/(leafNumList.get(deep-1)+1));                leafNumList.set(deep-1,leafNumList.get(deep-1)+1);            }            calculation(root.left,deep+1,deepList,leafNumList);            calculation(root.right,deep+1,deepList,leafNumList);        }    }    public List<Double> averageOfLevels(TreeNode root) {        //deepList存储每一层的平均值        List<Double> deepList= new ArrayList<>();        //leafNumList存储每一层叶子节点个数        List<Integer> leafNumList= new ArrayList<>();        calculation(root,1,deepList,leafNumList);        return deepList;    }}