LeetCode 637. Average of Levels in Binary Tree(二叉树每层节点的平均值)

题意：求二叉树每层节点的平均值。

分析：bfs层次遍历，每步循环一次性处理一整层。

/**
 * 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 len = q.size();
            double sum = 0;
            for(int i = 0; i < len; ++i){
                TreeNode *tmp = q.front();
                q.pop();
                sum += double(tmp -> val);
                if(tmp -> left != NULL) q.push(tmp -> left);
                if(tmp -> right != NULL) q.push(tmp -> right);
            }
            ans.push_back(sum / len);
        }
        return ans;
        
    }
};