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].

（1）思想1：使用queue ，对二叉树进行层遍历。

C++：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<double> averageOfLevels(TreeNode* root) {
        vector<double> res;
        queue<TreeNode*> q{{root}};
        while(!q.empty())
        {
            int n=q.size();
            double sum=0;
            for(int i=0;i<n;i++)
            {
                TreeNode *t=q.front();
                q.pop();
                sum=sum+t->val;
                if(t->left)
                    q.push(t->left);
                if(t->right)
                    q.push(t->right);
            }
            res.push_back(sum/n);
        }
        return res;
     
    }
};