LeetCode: 637 Average of Levels in Binary Tree

题目：

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:

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

代码：

 使用队列的先进先出的特点：

class Solution {
public:
    vector<double> averageOfLevels(TreeNode* root) {
        vector<double> ret;
        queue<TreeNode*> q;
        q.push(root);
        while (!q.empty()) {
            long long t = 0;
            int n = q.size();
            for (int i = 0; i < n; i++) {
                auto node = q.front();
                q.pop();
                if (node->left) q.push(node->left);
                if (node->right) q.push(node->right);
                t += node->val;
            }
            ret.push_back(double(t)/double(n));
        }
        return ret;
    }
};