LeetCode-637. Average of Levels in Binary Tree

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.

题目：对于给定的二叉树，计算每层节点的平均值，返回vector<double>

思路：如果动手做了二叉树的层次遍历，这道题就很简单，二叉树的遍历是得到一个二维的vector，因此只需要对每个vector求平均即可，即将原来的res.push_back(lvl)改为 ret.push_back(accumulate(lvl.begin(),lvl.end(),0.0)/lvl.size())，这是求平均值的代码。注意accumulate（）初始值设为0.0。

代码：

class Solution {
public:
    vector<double> averageOfLevels(TreeNode* root) {
        vector<double> ret;
        if(!root)
        	return ret;
        queue<TreeNode*> q;
		q.push(root);
		while(!q.empty()){
			vector<int> lvl;
			int size=q.size();
        	for(;size>0;size--){
            	TreeNode* node=q.front();
            	lvl.push_back(node->val);
            	q.pop();
            	if(node->left)
                	q.push(node->left);
            	if(node->right)
            		q.push(node->right);
        	}
        ret.push_back(accumulate(lvl.begin(),lvl.end(),0.0)/lvl.size());//更改的地方
		}
		return ret;	
    }
};

AC,beats99%