leetcode637+ 求二叉树每一层sum的average+BFS

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

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) {
        if(!root) return {};
        vector<double> res;
        queue<TreeNode*> Q;
        Q.push(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 += t->val;
                if(t->left) Q.push(t->left);
                if(t->right) Q.push(t->right);
            }
            res.push_back(sum/n);
        }
        return res;
    }
};