Average of Levels in Binary Tree(leetcode)

Average of Levels in Binary Tree

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题目

leetcode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   7Output: [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: The range of node’s value is in the range of 32-bit signed integer.

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解析

题目需要我们做的是求二叉树每层的平均数，这就需要我们用到层次遍历，一层一层遍历二叉树，得到每层相应节点的值，计算其平均值。
这样我们可以利用STL中的queue“先进先出”的特点，实现对二叉树的层次遍历，从而可以计算每层节点的平均数。

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解决

/** * 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> result;        if (root == NULL) return result;        queue<TreeNode *> tree;        tree.push(root);        // 判断是否完成层次遍历        while (!tree.empty()) {            int s = tree.size(); // 记录该层节点个数            double sum = 0;            for (int i = 0; i < s; i++) {            // 遍历该层的所有节点                TreeNode * temp = tree.front();                tree.pop();                // 计算该层节点值的总和                sum += temp->val;                // 为遍历下一层节点做准备                if (temp->left != NULL) {                    tree.push(temp->left);                }                if (temp->right != NULL) {                    tree.push(temp->right);                }            }            // 记录该层节点的平均值            result.push_back(sum / s);        }        return result;    }};