Average of Levels in Binary Tree

https://leetcode.com/problems/average-of-levels-in-binary-tree/#/description

求出二叉树每层的均值，这个就是层序遍历的变种，没什么好说的。只是test case 里有INT_MAX-1，这个就很烦了，我没想到什么好办法，就把数字全都专程double 来处理了。

思路用dfs 或者bfs 都可以，这里我采用了dfs，有个坑是给vector 赋值的时候是拷贝的，所以尽量用指针

/**
 * 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:
    void dfsIter(TreeNode *root, int level, vector<vector<double>*>& rets) {
        if (root == nullptr) return;

        vector<double> *l;
        if (rets.size() <= level) {
            l = new vector<double>();
            rets.push_back(l);
        } else {
            l = rets.at(level);
        }

        l->push_back(root->val);
        dfsIter(root->right, level+1, rets);
        dfsIter(root->left, level+1, rets);
    }
    vector<double> averageOfLevels(TreeNode* root) {
        if (root == nullptr) return vector<double>();
        vector<vector<double>*> levels;
        vector<double> result;
        dfsIter(root, 0, levels);
        for (auto level : levels) {
            double sum = 0;
            for (int n : *level) {
                sum += (double)n;
            }
            result.push_back((double)sum / (double)level->size());
        }
    
        return result;
    }
};