力扣637. 二叉树的层平均值

深度优先遍历

• 思路：
  • 使用深度优先搜索计算二叉树的层平均值，维护两个数组用于统计各层节点和、各层节点个数；
  • 递归统计时，需要传入当前统计深度；

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<double> averageOfLevels(TreeNode* root) {
        auto counts = std::vector<int>();
        auto sums = std::vector<double>();

        dfs(root, 0, counts, sums);

        int size = sums.size();
        auto averages = std::vector<double>();
        for (int i = 0; i < size; ++i) {
            averages.push_back(sums[i] / counts[i]);
        }

        return averages;
    }

    void dfs(TreeNode* root, int depth, std::vector<int>& counts, std::vector<double>& sums) {
        if (root == nullptr) {
            return;
        }

        if (depth < sums.size()) {
            sums[depth] += root->val;
            counts[depth] += 1;
        } else {
            sums.push_back(1.0 * root->val);
            counts.push_back(1);
        }
        
        dfs(root->left, depth + 1, counts, sums);
        dfs(root->right, depth + 1, counts, sums);
    }
};