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:

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

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public List<Double> averageOfLevels(TreeNode root) {
List<Double> result=new ArrayList<>();
Queue<TreeNode> q=new LinkedList<>();

q.add(root);
while(!q.isEmpty())
{
int n=q.size();//很巧妙的计算每层个数，只有使用完该层才会进入到下一层实验
double sum=0.0;
for(int i=0;i<n;i++)
{
TreeNode x=q.poll();
sum+=x.val;
if(x.left!=null)q.add(x.left);
if(x.right!=null)q.add(x.right);
}
result.add(sum/n);
}
return result;
}
}

一个简单的广度优先搜索，难点在于思考广度优先搜索每一层的个数鉴定