1120. Maximum Average Subtree

O(n) time O(h) space

/**
 * 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:
    double maximumAverageSubtree(TreeNode* root) {
        dfs(root);
        return res;
    }

    pair<int, int> dfs(TreeNode *root) {
        if (!root) return {0, 0};
        auto [ls, lc] = dfs(root->left);
        auto [rs, rc] = dfs(root->right);
        int s = root->val + ls + rs, c = 1 + lc + rc;
        res = max(res, double(s) / c);
        return {s, c};
    }

    double res = 0;
};