leetcode 637. Average of Levels in Binary Tree

1.题目

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].

注意每个节点上的值为32位有符号整数
Note:
The range of node’s value is in the range of 32-bit signed integer.

2.分析

广度优先遍历，对每一层的节点值求和，再取平均值。
注意：由于节点值32位整数，求和如果用int保存会溢出，而平均值是double类型。所以用double来保存求和

3.代码

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