637.cpp

/**
 * 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> ans;
        if (root == NULL) return ans;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty()) {
            int n = q.size();
            long long sum = 0;
            for(int i = 0; i < n; i++) {
                TreeNode* t = q.front();
                q.pop();
                if (t->left) q.push(t->left);
                if (t->right) q.push(t->right);
                sum += t->val;
            }
            ans.push_back(static_cast<double>(sum) / n);
        }
        return ans;
    }
};