计算二叉树每层平均值

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].

按照题目要求求解二叉树每层的平均值，既然按照层级求解，那么可以说是层次遍历或者说是BFS的典型问题。

关键在于如何正确的在遍历过程中分层，思路是根据压入队列的元素个数划分。代码如下：

/**
 * 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 averageOfLevels(TreeNode* root) {
        vector v;
        if (root == NULL)
            return v;
        queue q;
        q.push(root);
        while (!q.empty())
        {
            int num = q.size();
            long temp = 0;
            for (int i = 0; i < num; i++)
            {
                TreeNode *top = q.front();
                q.pop();
                if (top->left != NULL)
                    q.push(top->left);
                if (top->right != NULL)
                    q.push(top->right);
                temp += top->val;
            }
            v.push_back((double)temp / num);
        }
        return v;
    }
};